Adding upstream version 4.94.2.upstream/4.94.2upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 1878 insertions, 0 deletions

diff --git a/src/arc.c b/src/arc.c
new file mode 100644
index 0000000..391af07
--- /dev/null
+++ b/src/arc.c
@@ -0,0 +1,1878 @@
+/*************************************************
+*     Exim - an Internet mail transport agent    *
+*************************************************/
+/* Experimental ARC support for Exim
+   Copyright (c) Jeremy Harris 2018 - 2020
+   License: GPL
+*/
+
+#include "exim.h"
+#if defined EXPERIMENTAL_ARC
+# if defined DISABLE_DKIM
+#  error DKIM must also be enabled for ARC
+# else
+
+#  include "functions.h"
+#  include "pdkim/pdkim.h"
+#  include "pdkim/signing.h"
+
+extern pdkim_ctx * dkim_verify_ctx;
+extern pdkim_ctx dkim_sign_ctx;
+
+#define ARC_SIGN_OPT_TSTAMP	BIT(0)
+#define ARC_SIGN_OPT_EXPIRE	BIT(1)
+
+#define ARC_SIGN_DEFAULT_EXPIRE_DELTA (60 * 60 * 24 * 30)	/* one month */
+
+/******************************************************************************/
+
+typedef struct hdr_rlist {
+  struct hdr_rlist *	prev;
+  BOOL			used;
+  header_line *		h;
+} hdr_rlist;
+
+typedef struct arc_line {
+  header_line *	complete;	/* including the header name; nul-term */
+  uschar *	relaxed;
+
+  /* identified tag contents */
+  /*XXX t= for AS? */
+  blob		i;
+  blob		cv;
+  blob		a;
+  blob		b;
+  blob		bh;
+  blob		d;
+  blob		h;
+  blob		s;
+  blob		c;
+  blob		l;
+
+  /* tag content sub-portions */
+  blob		a_algo;
+  blob		a_hash;
+
+  blob		c_head;
+  blob		c_body;
+
+  /* modified copy of b= field in line */
+  blob		rawsig_no_b_val;
+} arc_line;
+
+typedef struct arc_set {
+  struct arc_set *	next;
+  struct arc_set *	prev;
+
+  unsigned		instance;
+  arc_line *		hdr_aar;
+  arc_line *		hdr_ams;
+  arc_line *		hdr_as;
+
+  const uschar *	ams_verify_done;
+  BOOL			ams_verify_passed;
+} arc_set;
+
+typedef struct arc_ctx {
+  arc_set *	arcset_chain;
+  arc_set *	arcset_chain_last;
+} arc_ctx;
+
+#define ARC_HDR_AAR	US"ARC-Authentication-Results:"
+#define ARC_HDRLEN_AAR	27
+#define ARC_HDR_AMS	US"ARC-Message-Signature:"
+#define ARC_HDRLEN_AMS	22
+#define ARC_HDR_AS	US"ARC-Seal:"
+#define ARC_HDRLEN_AS	9
+#define HDR_AR		US"Authentication-Results:"
+#define HDRLEN_AR	23
+
+static time_t now;
+static time_t expire;
+static hdr_rlist * headers_rlist;
+static arc_ctx arc_sign_ctx = { NULL };
+static arc_ctx arc_verify_ctx = { NULL };
+
+
+/******************************************************************************/
+
+
+/* Get the instance number from the header.
+Return 0 on error */
+static unsigned
+arc_instance_from_hdr(const arc_line * al)
+{
+const uschar * s = al->i.data;
+if (!s || !al->i.len) return 0;
+return (unsigned) atoi(CCS s);
+}
+
+
+static uschar *
+skip_fws(uschar * s)
+{
+uschar c = *s;
+while (c && (c == ' ' || c == '\t' || c == '\n' || c == '\r')) c = *++s;
+return s;
+}
+
+
+/* Locate instance struct on chain, inserting a new one if
+needed.  The chain is in increasing-instance-number order
+by the "next" link, and we have a "prev" link also.
+*/
+
+static arc_set *
+arc_find_set(arc_ctx * ctx, unsigned i)
+{
+arc_set ** pas, * as, * next, * prev;
+
+for (pas = &ctx->arcset_chain, prev = NULL, next = ctx->arcset_chain;
+     as = *pas; pas = &as->next)
+  {
+  if (as->instance > i) break;
+  if (as->instance == i)
+    {
+    DEBUG(D_acl) debug_printf("ARC: existing instance %u\n", i);
+    return as;
+    }
+  next = as->next;
+  prev = as;
+  }
+
+DEBUG(D_acl) debug_printf("ARC: new instance %u\n", i);
+*pas = as = store_get(sizeof(arc_set), FALSE);
+memset(as, 0, sizeof(arc_set));
+as->next = next;
+as->prev = prev;
+as->instance = i;
+if (next)
+  next->prev = as;
+else
+  ctx->arcset_chain_last = as;
+return as;
+}
+
+
+
+/* Insert a tag content into the line structure.
+Note this is a reference to existing data, not a copy.
+Check for already-seen tag.
+The string-pointer is on the '=' for entry.  Update it past the
+content (to the ;) on return;
+*/
+
+static uschar *
+arc_insert_tagvalue(arc_line * al, unsigned loff, uschar ** ss)
+{
+uschar * s = *ss;
+uschar c = *++s;
+blob * b = (blob *)(US al + loff);
+size_t len = 0;
+
+/* [FWS] tag-value [FWS] */
+
+if (b->data) return US"fail";
+s = skip_fws(s);						/* FWS */
+
+b->data = s;
+while ((c = *s) && c != ';') { len++; s++; }
+*ss = s;
+while (len && ((c = s[-1]) == ' ' || c == '\t' || c == '\n' || c == '\r'))
+  { s--; len--; }						/* FWS */
+b->len = len;
+return NULL;
+}
+
+
+/* Inspect a header line, noting known tag fields.
+Check for duplicates. */
+
+static uschar *
+arc_parse_line(arc_line * al, header_line * h, unsigned off, BOOL instance_only)
+{
+uschar * s = h->text + off;
+uschar * r = NULL;	/* compiler-quietening */
+uschar c;
+
+al->complete = h;
+
+if (!instance_only)
+  {
+  al->rawsig_no_b_val.data = store_get(h->slen + 1, TRUE);	/* tainted */
+  memcpy(al->rawsig_no_b_val.data, h->text, off);	/* copy the header name blind */
+  r = al->rawsig_no_b_val.data + off;
+  al->rawsig_no_b_val.len = off;
+  }
+
+/* tag-list  =  tag-spec *( ";" tag-spec ) [ ";" ] */
+
+while ((c = *s))
+  {
+  char tagchar;
+  uschar * t;
+  unsigned i = 0;
+  uschar * fieldstart = s;
+  uschar * bstart = NULL, * bend;
+
+  /* tag-spec  =  [FWS] tag-name [FWS] "=" [FWS] tag-value [FWS] */
+
+  s = skip_fws(s);						/* FWS */
+  if (!*s) break;
+/* debug_printf("%s: consider '%s'\n", __FUNCTION__, s); */
+  tagchar = *s++;
+  s = skip_fws(s);						/* FWS */
+  if (!*s) break;
+
+  if (!instance_only || tagchar == 'i') switch (tagchar)
+    {
+    case 'a':				/* a= AMS algorithm */
+      {
+      if (*s != '=') return US"no 'a' value";
+      if (arc_insert_tagvalue(al, offsetof(arc_line, a), &s)) return US"a tag dup";
+
+      /* substructure: algo-hash   (eg. rsa-sha256) */
+
+      t = al->a_algo.data = al->a.data;
+      while (*t != '-')
+	if (!*t++ || ++i > al->a.len) return US"no '-' in 'a' value";
+      al->a_algo.len = i;
+      if (*t++ != '-') return US"no '-' in 'a' value";
+      al->a_hash.data = t;
+      al->a_hash.len = al->a.len - i - 1;
+      }
+      break;
+    case 'b':
+      {
+      gstring * g = NULL;
+
+      switch (*s)
+	{
+	case '=':			/* b= AMS signature */
+	  if (al->b.data) return US"already b data";
+	  bstart = s+1;
+
+	  /* The signature can have FWS inserted in the content;
+	  make a stripped copy */
+
+	  while ((c = *++s) && c != ';')
+	    if (c != ' ' && c != '\t' && c != '\n' && c != '\r')
+	      g = string_catn(g, s, 1);
+	  if (!g) return US"no b= value";
+	  al->b.data = string_from_gstring(g);
+	  al->b.len = g->ptr;
+	  gstring_release_unused(g);
+	  bend = s;
+	  break;
+	case 'h':			/* bh= AMS body hash */
+	  s = skip_fws(++s);					/* FWS */
+	  if (*s != '=') return US"no bh value";
+	  if (al->bh.data) return US"already bh data";
+
+	  /* The bodyhash can have FWS inserted in the content;
+	  make a stripped copy */
+
+	  while ((c = *++s) && c != ';')
+	    if (c != ' ' && c != '\t' && c != '\n' && c != '\r')
+	      g = string_catn(g, s, 1);
+	  if (!g) return US"no bh= value";
+	  al->bh.data = string_from_gstring(g);
+	  al->bh.len = g->ptr;
+	  gstring_release_unused(g);
+	  break;
+	default:
+	  return US"b? tag";
+	}
+      }
+      break;
+    case 'c':
+      switch (*s)
+	{
+	case '=':			/* c= AMS canonicalisation */
+	  if (arc_insert_tagvalue(al, offsetof(arc_line, c), &s)) return US"c tag dup";
+
+	  /* substructure: head/body   (eg. relaxed/simple)) */
+
+	  t = al->c_head.data = al->c.data;
+	  while (isalpha(*t))
+	    if (!*t++ || ++i > al->a.len) break;
+	  al->c_head.len = i;
+	  if (*t++ == '/')		/* /body is optional */
+	    {
+	    al->c_body.data = t;
+	    al->c_body.len = al->c.len - i - 1;
+	    }
+	  else
+	    {
+	    al->c_body.data = US"simple";
+	    al->c_body.len = 6;
+	    }
+	  break;
+	case 'v':			/* cv= AS validity */
+	  if (*++s != '=') return US"cv tag val";
+	  if (arc_insert_tagvalue(al, offsetof(arc_line, cv), &s)) return US"cv tag dup";
+	  break;
+	default:
+	  return US"c? tag";
+	}
+      break;
+    case 'd':				/* d= AMS domain */
+      if (*s != '=') return US"d tag val";
+      if (arc_insert_tagvalue(al, offsetof(arc_line, d), &s)) return US"d tag dup";
+      break;
+    case 'h':				/* h= AMS headers */
+      if (*s != '=') return US"h tag val";
+      if (arc_insert_tagvalue(al, offsetof(arc_line, h), &s)) return US"h tag dup";
+      break;
+    case 'i':				/* i= ARC set instance */
+      if (*s != '=') return US"i tag val";
+      if (arc_insert_tagvalue(al, offsetof(arc_line, i), &s)) return US"i tag dup";
+      if (instance_only) goto done;
+      break;
+    case 'l':				/* l= bodylength */
+      if (*s != '=') return US"l tag val";
+      if (arc_insert_tagvalue(al, offsetof(arc_line, l), &s)) return US"l tag dup";
+      break;
+    case 's':				/* s= AMS selector */
+      if (*s != '=') return US"s tag val";
+      if (arc_insert_tagvalue(al, offsetof(arc_line, s), &s)) return US"s tag dup";
+      break;
+    }
+
+  while ((c = *s) && c != ';') s++;
+  if (c) s++;				/* ; after tag-spec */
+
+  /* for all but the b= tag, copy the field including FWS.  For the b=,
+  drop the tag content. */
+
+  if (!instance_only)
+    if (bstart)
+      {
+      size_t n = bstart - fieldstart;
+      memcpy(r, fieldstart, n);		/* FWS "b=" */
+      r += n;
+      al->rawsig_no_b_val.len += n;
+      n = s - bend;
+      memcpy(r, bend, n);		/* FWS ";" */
+      r += n;
+      al->rawsig_no_b_val.len += n;
+      }
+    else
+      {
+      size_t n = s - fieldstart;
+      memcpy(r, fieldstart, n);
+      r += n;
+      al->rawsig_no_b_val.len += n;
+      }
+  }
+
+if (!instance_only)
+  *r = '\0';
+
+done:
+/* debug_printf("%s: finshed\n", __FUNCTION__); */
+return NULL;
+}
+
+
+/* Insert one header line in the correct set of the chain,
+adding instances as needed and checking for duplicate lines.
+*/
+
+static uschar *
+arc_insert_hdr(arc_ctx * ctx, header_line * h, unsigned off, unsigned hoff,
+  BOOL instance_only, arc_line ** alp_ret)
+{
+unsigned i;
+arc_set * as;
+arc_line * al = store_get(sizeof(arc_line), FALSE), ** alp;
+uschar * e;
+
+memset(al, 0, sizeof(arc_line));
+
+if ((e = arc_parse_line(al, h, off, instance_only)))
+  {
+  DEBUG(D_acl) if (e) debug_printf("ARC: %s\n", e);
+  return US"line parse";
+  }
+if (!(i = arc_instance_from_hdr(al)))	return US"instance find";
+if (i > 50)				return US"overlarge instance number";
+if (!(as = arc_find_set(ctx, i)))	return US"set find";
+if (*(alp = (arc_line **)(US as + hoff))) return US"dup hdr";
+
+*alp = al;
+if (alp_ret) *alp_ret = al;
+return NULL;
+}
+
+
+
+
+static const uschar *
+arc_try_header(arc_ctx * ctx, header_line * h, BOOL instance_only)
+{
+const uschar * e;
+
+/*debug_printf("consider hdr '%s'\n", h->text);*/
+if (strncmpic(ARC_HDR_AAR, h->text, ARC_HDRLEN_AAR) == 0)
+  {
+  DEBUG(D_acl)
+    {
+    int len = h->slen;
+    uschar * s;
+    for (s = h->text + h->slen; s[-1] == '\r' || s[-1] == '\n'; )
+      s--, len--;
+    debug_printf("ARC: found AAR: %.*s\n", len, h->text);
+    }
+  if ((e = arc_insert_hdr(ctx, h, ARC_HDRLEN_AAR, offsetof(arc_set, hdr_aar),
+			  TRUE, NULL)))
+    {
+    DEBUG(D_acl) debug_printf("inserting AAR: %s\n", e);
+    return US"inserting AAR";
+    }
+  }
+else if (strncmpic(ARC_HDR_AMS, h->text, ARC_HDRLEN_AMS) == 0)
+  {
+  arc_line * ams;
+
+  DEBUG(D_acl)
+    {
+    int len = h->slen;
+    uschar * s;
+    for (s = h->text + h->slen; s[-1] == '\r' || s[-1] == '\n'; )
+      s--, len--;
+    debug_printf("ARC: found AMS: %.*s\n", len, h->text);
+    }
+  if ((e = arc_insert_hdr(ctx, h, ARC_HDRLEN_AMS, offsetof(arc_set, hdr_ams),
+			  instance_only, &ams)))
+    {
+    DEBUG(D_acl) debug_printf("inserting AMS: %s\n", e);
+    return US"inserting AMS";
+    }
+
+  /* defaults */
+  if (!ams->c.data)
+    {
+    ams->c_head.data = US"simple"; ams->c_head.len = 6;
+    ams->c_body = ams->c_head;
+    }
+  }
+else if (strncmpic(ARC_HDR_AS, h->text, ARC_HDRLEN_AS) == 0)
+  {
+  DEBUG(D_acl)
+    {
+    int len = h->slen;
+    uschar * s;
+    for (s = h->text + h->slen; s[-1] == '\r' || s[-1] == '\n'; )
+      s--, len--;
+    debug_printf("ARC: found AS: %.*s\n", len, h->text);
+    }
+  if ((e = arc_insert_hdr(ctx, h, ARC_HDRLEN_AS, offsetof(arc_set, hdr_as),
+			  instance_only, NULL)))
+    {
+    DEBUG(D_acl) debug_printf("inserting AS: %s\n", e);
+    return US"inserting AS";
+    }
+  }
+return NULL;
+}
+
+
+
+/* Gather the chain of arc sets from the headers.
+Check for duplicates while that is done.  Also build the
+reverse-order headers list;
+
+Return: ARC state if determined, eg. by lack of any ARC chain.
+*/
+
+static const uschar *
+arc_vfy_collect_hdrs(arc_ctx * ctx)
+{
+header_line * h;
+hdr_rlist * r = NULL, * rprev = NULL;
+const uschar * e;
+
+DEBUG(D_acl) debug_printf("ARC: collecting arc sets\n");
+for (h = header_list; h; h = h->next)
+  {
+  r = store_get(sizeof(hdr_rlist), FALSE);
+  r->prev = rprev;
+  r->used = FALSE;
+  r->h = h;
+  rprev = r;
+
+  if ((e = arc_try_header(ctx, h, FALSE)))
+    {
+    arc_state_reason = string_sprintf("collecting headers: %s", e);
+    return US"fail";
+    }
+  }
+headers_rlist = r;
+
+if (!ctx->arcset_chain) return US"none";
+return NULL;
+}
+
+
+static BOOL
+arc_cv_match(arc_line * al, const uschar * s)
+{
+return Ustrncmp(s, al->cv.data, al->cv.len) == 0;
+}
+
+/******************************************************************************/
+
+/* Return the hash of headers from the message that the AMS claims it
+signed.
+*/
+
+static void
+arc_get_verify_hhash(arc_ctx * ctx, arc_line * ams, blob * hhash)
+{
+const uschar * headernames = string_copyn(ams->h.data, ams->h.len);
+const uschar * hn;
+int sep = ':';
+hdr_rlist * r;
+BOOL relaxed = Ustrncmp(US"relaxed", ams->c_head.data, ams->c_head.len) == 0;
+int hashtype = pdkim_hashname_to_hashtype(
+		    ams->a_hash.data, ams->a_hash.len);
+hctx hhash_ctx;
+const uschar * s;
+int len;
+
+if (  hashtype == -1
+   || !exim_sha_init(&hhash_ctx, pdkim_hashes[hashtype].exim_hashmethod))
+  {
+  DEBUG(D_acl)
+      debug_printf("ARC: hash setup error, possibly nonhandled hashtype\n");
+  return;
+  }
+
+/* For each headername in the list from the AMS (walking in order)
+walk the message headers in reverse order, adding to the hash any
+found for the first time. For that last point, maintain used-marks
+on the list of message headers. */
+
+DEBUG(D_acl) debug_printf("ARC: AMS header data for verification:\n");
+
+for (r = headers_rlist; r; r = r->prev)
+  r->used = FALSE;
+while ((hn = string_nextinlist(&headernames, &sep, NULL, 0)))
+  for (r = headers_rlist; r; r = r->prev)
+    if (  !r->used
+       && strncasecmp(CCS (s = r->h->text), CCS hn, Ustrlen(hn)) == 0
+       )
+      {
+      if (relaxed) s = pdkim_relax_header_n(s, r->h->slen, TRUE);
+
+      len = Ustrlen(s);
+      DEBUG(D_acl) pdkim_quoteprint(s, len);
+      exim_sha_update(&hhash_ctx, s, Ustrlen(s));
+      r->used = TRUE;
+      break;
+      }
+
+/* Finally add in the signature header (with the b= tag stripped); no CRLF */
+
+s = ams->rawsig_no_b_val.data, len = ams->rawsig_no_b_val.len;
+if (relaxed)
+  len = Ustrlen(s = pdkim_relax_header_n(s, len, FALSE));
+DEBUG(D_acl) pdkim_quoteprint(s, len);
+exim_sha_update(&hhash_ctx, s, len);
+
+exim_sha_finish(&hhash_ctx, hhash);
+DEBUG(D_acl)
+  { debug_printf("ARC: header hash: "); pdkim_hexprint(hhash->data, hhash->len); }
+return;
+}
+
+
+
+
+static pdkim_pubkey *
+arc_line_to_pubkey(arc_line * al)
+{
+uschar * dns_txt;
+pdkim_pubkey * p;
+
+if (!(dns_txt = dkim_exim_query_dns_txt(string_sprintf("%.*s._domainkey.%.*s",
+	  (int)al->s.len, al->s.data, (int)al->d.len, al->d.data))))
+  {
+  DEBUG(D_acl) debug_printf("pubkey dns lookup fail\n");
+  return NULL;
+  }
+
+if (  !(p = pdkim_parse_pubkey_record(dns_txt))
+   || (Ustrcmp(p->srvtype, "*") != 0 && Ustrcmp(p->srvtype, "email") != 0)
+   )
+  {
+  DEBUG(D_acl) debug_printf("pubkey dns lookup format error\n");
+  return NULL;
+  }
+
+/* If the pubkey limits use to specified hashes, reject unusable
+signatures. XXX should we have looked for multiple dns records? */
+
+if (p->hashes)
+  {
+  const uschar * list = p->hashes, * ele;
+  int sep = ':';
+
+  while ((ele = string_nextinlist(&list, &sep, NULL, 0)))
+    if (Ustrncmp(ele, al->a_hash.data, al->a_hash.len) == 0) break;
+  if (!ele)
+    {
+    DEBUG(D_acl) debug_printf("pubkey h=%s vs sig a=%.*s\n",
+			      p->hashes, (int)al->a.len, al->a.data);
+    return NULL;
+    }
+  }
+return p;
+}
+
+
+
+
+static pdkim_bodyhash *
+arc_ams_setup_vfy_bodyhash(arc_line * ams)
+{
+int canon_head = -1, canon_body = -1;
+long bodylen;
+
+if (!ams->c.data) ams->c.data = US"simple";	/* RFC 6376 (DKIM) default */
+pdkim_cstring_to_canons(ams->c.data, ams->c.len, &canon_head, &canon_body);
+bodylen = ams->l.data
+	? strtol(CS string_copyn(ams->l.data, ams->l.len), NULL, 10) : -1;
+
+return pdkim_set_bodyhash(dkim_verify_ctx,
+	pdkim_hashname_to_hashtype(ams->a_hash.data, ams->a_hash.len),
+	canon_body,
+	bodylen);
+}
+
+
+
+/* Verify an AMS. This is a DKIM-sig header, but with an ARC i= tag
+and without a DKIM v= tag.
+*/
+
+static const uschar *
+arc_ams_verify(arc_ctx * ctx, arc_set * as)
+{
+arc_line * ams = as->hdr_ams;
+pdkim_bodyhash * b;
+pdkim_pubkey * p;
+blob sighash;
+blob hhash;
+ev_ctx vctx;
+int hashtype;
+const uschar * errstr;
+
+as->ams_verify_done = US"in-progress";
+
+/* Check the AMS has all the required tags:
+   "a="  algorithm
+   "b="  signature
+   "bh=" body hash
+   "d="  domain (for key lookup)
+   "h="  headers (included in signature)
+   "s="  key-selector (for key lookup)
+*/
+if (  !ams->a.data || !ams->b.data || !ams->bh.data || !ams->d.data
+   || !ams->h.data || !ams->s.data)
+  {
+  as->ams_verify_done = arc_state_reason = US"required tag missing";
+  return US"fail";
+  }
+
+
+/* The bodyhash should have been created earlier, and the dkim code should
+have managed calculating it during message input.  Find the reference to it. */
+
+if (!(b = arc_ams_setup_vfy_bodyhash(ams)))
+  {
+  as->ams_verify_done = arc_state_reason = US"internal hash setup error";
+  return US"fail";
+  }
+
+DEBUG(D_acl)
+  {
+  debug_printf("ARC i=%d AMS   Body bytes hashed: %lu\n"
+	       "              Body %.*s computed: ",
+	       as->instance, b->signed_body_bytes,
+	       (int)ams->a_hash.len, ams->a_hash.data);
+  pdkim_hexprint(CUS b->bh.data, b->bh.len);
+  }
+
+/* We know the bh-tag blob is of a nul-term string, so safe as a string */
+
+if (  !ams->bh.data
+   || (pdkim_decode_base64(ams->bh.data, &sighash), sighash.len != b->bh.len)
+   || memcmp(sighash.data, b->bh.data, b->bh.len) != 0
+   )
+  {
+  DEBUG(D_acl)
+    {
+    debug_printf("ARC i=%d AMS Body hash from headers: ", as->instance);
+    pdkim_hexprint(sighash.data, sighash.len);
+    debug_printf("ARC i=%d AMS Body hash did NOT match\n", as->instance);
+    }
+  return as->ams_verify_done = arc_state_reason = US"AMS body hash miscompare";
+  }
+
+DEBUG(D_acl) debug_printf("ARC i=%d AMS Body hash compared OK\n", as->instance);
+
+/* Get the public key from DNS */
+
+if (!(p = arc_line_to_pubkey(ams)))
+  return as->ams_verify_done = arc_state_reason = US"pubkey problem";
+
+/* We know the b-tag blob is of a nul-term string, so safe as a string */
+pdkim_decode_base64(ams->b.data, &sighash);
+
+arc_get_verify_hhash(ctx, ams, &hhash);
+
+/* Setup the interface to the signing library */
+
+if ((errstr = exim_dkim_verify_init(&p->key, KEYFMT_DER, &vctx, NULL)))
+  {
+  DEBUG(D_acl) debug_printf("ARC verify init: %s\n", errstr);
+  as->ams_verify_done = arc_state_reason = US"internal sigverify init error";
+  return US"fail";
+  }
+
+hashtype = pdkim_hashname_to_hashtype(ams->a_hash.data, ams->a_hash.len);
+if (hashtype == -1)
+  {
+  DEBUG(D_acl) debug_printf("ARC i=%d AMS verify bad a_hash\n", as->instance);
+  return as->ams_verify_done = arc_state_reason = US"AMS sig nonverify";
+  }
+
+if ((errstr = exim_dkim_verify(&vctx,
+	  pdkim_hashes[hashtype].exim_hashmethod, &hhash, &sighash)))
+  {
+  DEBUG(D_acl) debug_printf("ARC i=%d AMS verify %s\n", as->instance, errstr);
+  return as->ams_verify_done = arc_state_reason = US"AMS sig nonverify";
+  }
+
+DEBUG(D_acl) debug_printf("ARC i=%d AMS verify pass\n", as->instance);
+as->ams_verify_passed = TRUE;
+return NULL;
+}
+
+
+
+/* Check the sets are instance-continuous and that all
+members are present.  Check that no arc_seals are "fail".
+Set the highest instance number global.
+Verify the latest AMS.
+*/
+static uschar *
+arc_headers_check(arc_ctx * ctx)
+{
+arc_set * as;
+int inst;
+BOOL ams_fail_found = FALSE;
+
+if (!(as = ctx->arcset_chain_last))
+  return US"none";
+
+for(inst = as->instance; as; as = as->prev, inst--)
+  {
+  if (as->instance != inst)
+    arc_state_reason = string_sprintf("i=%d (sequence; expected %d)",
+      as->instance, inst);
+  else if (!as->hdr_aar || !as->hdr_ams || !as->hdr_as)
+    arc_state_reason = string_sprintf("i=%d (missing header)", as->instance);
+  else if (arc_cv_match(as->hdr_as, US"fail"))
+    arc_state_reason = string_sprintf("i=%d (cv)", as->instance);
+  else
+    goto good;
+
+  DEBUG(D_acl) debug_printf("ARC chain fail at %s\n", arc_state_reason);
+  return US"fail";
+
+  good:
+  /* Evaluate the oldest-pass AMS validation while we're here.
+  It does not affect the AS chain validation but is reported as
+  auxilary info. */
+
+  if (!ams_fail_found)
+    if (arc_ams_verify(ctx, as))
+      ams_fail_found = TRUE;
+    else
+      arc_oldest_pass = inst;
+  arc_state_reason = NULL;
+  }
+if (inst != 0)
+  {
+  arc_state_reason = string_sprintf("(sequence; expected i=%d)", inst);
+  DEBUG(D_acl) debug_printf("ARC chain fail %s\n", arc_state_reason);
+  return US"fail";
+  }
+
+arc_received = ctx->arcset_chain_last;
+arc_received_instance = arc_received->instance;
+
+/* We can skip the latest-AMS validation, if we already did it. */
+
+as = ctx->arcset_chain_last;
+if (!as->ams_verify_passed)
+  {
+  if (as->ams_verify_done)
+    {
+    arc_state_reason = as->ams_verify_done;
+    return US"fail";
+    }
+  if (!!arc_ams_verify(ctx, as))
+    return US"fail";
+  }
+return NULL;
+}
+
+
+/******************************************************************************/
+static const uschar *
+arc_seal_verify(arc_ctx * ctx, arc_set * as)
+{
+arc_line * hdr_as = as->hdr_as;
+arc_set * as2;
+int hashtype;
+hctx hhash_ctx;
+blob hhash_computed;
+blob sighash;
+ev_ctx vctx;
+pdkim_pubkey * p;
+const uschar * errstr;
+
+DEBUG(D_acl) debug_printf("ARC: AS vfy i=%d\n", as->instance);
+/*
+       1.  If the value of the "cv" tag on that seal is "fail", the
+           chain state is "fail" and the algorithm stops here.  (This
+           step SHOULD be skipped if the earlier step (2.1) was
+           performed) [it was]
+
+       2.  In Boolean nomenclature: if ((i == 1 && cv != "none") or (cv
+           == "none" && i != 1)) then the chain state is "fail" and the
+           algorithm stops here (note that the ordering of the logic is
+           structured for short-circuit evaluation).
+*/
+
+if (  as->instance == 1 && !arc_cv_match(hdr_as, US"none")
+   || arc_cv_match(hdr_as, US"none") && as->instance != 1
+   )
+  {
+  arc_state_reason = US"seal cv state";
+  return US"fail";
+  }
+
+/*
+       3.  Initialize a hash function corresponding to the "a" tag of
+           the ARC-Seal.
+*/
+
+hashtype = pdkim_hashname_to_hashtype(hdr_as->a_hash.data, hdr_as->a_hash.len);
+
+if (  hashtype == -1
+   || !exim_sha_init(&hhash_ctx, pdkim_hashes[hashtype].exim_hashmethod))
+  {
+  DEBUG(D_acl)
+      debug_printf("ARC: hash setup error, possibly nonhandled hashtype\n");
+  arc_state_reason = US"seal hash setup error";
+  return US"fail";
+  }
+
+/*
+       4.  Compute the canonicalized form of the ARC header fields, in
+           the order described in Section 5.4.2, using the "relaxed"
+           header canonicalization defined in Section 3.4.2 of
+           [RFC6376].  Pass the canonicalized result to the hash
+           function.
+
+Headers are CRLF-separated, but the last one is not crlf-terminated.
+*/
+
+DEBUG(D_acl) debug_printf("ARC: AS header data for verification:\n");
+for (as2 = ctx->arcset_chain;
+     as2 && as2->instance <= as->instance;
+     as2 = as2->next)
+  {
+  arc_line * al;
+  uschar * s;
+  int len;
+
+  al = as2->hdr_aar;
+  if (!(s = al->relaxed))
+    al->relaxed = s = pdkim_relax_header_n(al->complete->text,
+					    al->complete->slen, TRUE);
+  len = Ustrlen(s);
+  DEBUG(D_acl) pdkim_quoteprint(s, len);
+  exim_sha_update(&hhash_ctx, s, len);
+
+  al = as2->hdr_ams;
+  if (!(s = al->relaxed))
+    al->relaxed = s = pdkim_relax_header_n(al->complete->text,
+					    al->complete->slen, TRUE);
+  len = Ustrlen(s);
+  DEBUG(D_acl) pdkim_quoteprint(s, len);
+  exim_sha_update(&hhash_ctx, s, len);
+
+  al = as2->hdr_as;
+  if (as2->instance == as->instance)
+    s = pdkim_relax_header_n(al->rawsig_no_b_val.data,
+					al->rawsig_no_b_val.len, FALSE);
+  else if (!(s = al->relaxed))
+    al->relaxed = s = pdkim_relax_header_n(al->complete->text,
+					    al->complete->slen, TRUE);
+  len = Ustrlen(s);
+  DEBUG(D_acl) pdkim_quoteprint(s, len);
+  exim_sha_update(&hhash_ctx, s, len);
+  }
+
+/*
+       5.  Retrieve the final digest from the hash function.
+*/
+
+exim_sha_finish(&hhash_ctx, &hhash_computed);
+DEBUG(D_acl)
+  {
+  debug_printf("ARC i=%d AS Header %.*s computed: ",
+    as->instance, (int)hdr_as->a_hash.len, hdr_as->a_hash.data);
+  pdkim_hexprint(hhash_computed.data, hhash_computed.len);
+  }
+
+
+/*
+       6.  Retrieve the public key identified by the "s" and "d" tags in
+           the ARC-Seal, as described in Section 4.1.6.
+*/
+
+if (!(p = arc_line_to_pubkey(hdr_as)))
+  return US"pubkey problem";
+
+/*
+       7.  Determine whether the signature portion ("b" tag) of the ARC-
+           Seal and the digest computed above are valid according to the
+           public key.  (See also Section Section 8.4 for failure case
+           handling)
+
+       8.  If the signature is not valid, the chain state is "fail" and
+           the algorithm stops here.
+*/
+
+/* We know the b-tag blob is of a nul-term string, so safe as a string */
+pdkim_decode_base64(hdr_as->b.data, &sighash);
+
+if ((errstr = exim_dkim_verify_init(&p->key, KEYFMT_DER, &vctx, NULL)))
+  {
+  DEBUG(D_acl) debug_printf("ARC verify init: %s\n", errstr);
+  return US"fail";
+  }
+
+if ((errstr = exim_dkim_verify(&vctx,
+	      pdkim_hashes[hashtype].exim_hashmethod,
+	      &hhash_computed, &sighash)))
+  {
+  DEBUG(D_acl)
+    debug_printf("ARC i=%d AS headers verify: %s\n", as->instance, errstr);
+  arc_state_reason = US"seal sigverify error";
+  return US"fail";
+  }
+
+DEBUG(D_acl) debug_printf("ARC: AS vfy i=%d pass\n", as->instance);
+return NULL;
+}
+
+
+static const uschar *
+arc_verify_seals(arc_ctx * ctx)
+{
+arc_set * as = ctx->arcset_chain_last;
+
+if (!as)
+  return US"none";
+
+for ( ; as; as = as->prev) if (arc_seal_verify(ctx, as)) return US"fail";
+
+DEBUG(D_acl) debug_printf("ARC: AS vfy overall pass\n");
+return NULL;
+}
+/******************************************************************************/
+
+/* Do ARC verification.  Called from DATA ACL, on a verify = arc
+condition.  No arguments; we are checking globals.
+
+Return:  The ARC state, or NULL on error.
+*/
+
+const uschar *
+acl_verify_arc(void)
+{
+const uschar * res;
+
+memset(&arc_verify_ctx, 0, sizeof(arc_verify_ctx));
+
+if (!dkim_verify_ctx)
+  {
+  DEBUG(D_acl) debug_printf("ARC: no DKIM verify context\n");
+  return NULL;
+  }
+
+/* AS evaluation, per
+https://tools.ietf.org/html/draft-ietf-dmarc-arc-protocol-10#section-6
+*/
+/* 1.  Collect all ARC sets currently on the message.  If there were
+       none, the ARC state is "none" and the algorithm stops here.
+*/
+
+if ((res = arc_vfy_collect_hdrs(&arc_verify_ctx)))
+  goto out;
+
+/* 2.  If the form of any ARC set is invalid (e.g., does not contain
+       exactly one of each of the three ARC-specific header fields),
+       then the chain state is "fail" and the algorithm stops here.
+
+       1.  To avoid the overhead of unnecessary computation and delay
+           from crypto and DNS operations, the cv value for all ARC-
+           Seal(s) MAY be checked at this point.  If any of the values
+           are "fail", then the overall state of the chain is "fail" and
+           the algorithm stops here.
+
+   3.  Conduct verification of the ARC-Message-Signature header field
+       bearing the highest instance number.  If this verification fails,
+       then the chain state is "fail" and the algorithm stops here.
+*/
+
+if ((res = arc_headers_check(&arc_verify_ctx)))
+  goto out;
+
+/* 4.  For each ARC-Seal from the "N"th instance to the first, apply the
+       following logic:
+
+       1.  If the value of the "cv" tag on that seal is "fail", the
+           chain state is "fail" and the algorithm stops here.  (This
+           step SHOULD be skipped if the earlier step (2.1) was
+           performed)
+
+       2.  In Boolean nomenclature: if ((i == 1 && cv != "none") or (cv
+           == "none" && i != 1)) then the chain state is "fail" and the
+           algorithm stops here (note that the ordering of the logic is
+           structured for short-circuit evaluation).
+
+       3.  Initialize a hash function corresponding to the "a" tag of
+           the ARC-Seal.
+
+       4.  Compute the canonicalized form of the ARC header fields, in
+           the order described in Section 5.4.2, using the "relaxed"
+           header canonicalization defined in Section 3.4.2 of
+           [RFC6376].  Pass the canonicalized result to the hash
+           function.
+
+       5.  Retrieve the final digest from the hash function.
+
+       6.  Retrieve the public key identified by the "s" and "d" tags in
+           the ARC-Seal, as described in Section 4.1.6.
+
+       7.  Determine whether the signature portion ("b" tag) of the ARC-
+           Seal and the digest computed above are valid according to the
+           public key.  (See also Section Section 8.4 for failure case
+           handling)
+
+       8.  If the signature is not valid, the chain state is "fail" and
+           the algorithm stops here.
+
+   5.  If all seals pass validation, then the chain state is "pass", and
+       the algorithm is complete.
+*/
+
+if ((res = arc_verify_seals(&arc_verify_ctx)))
+  goto out;
+
+res = US"pass";
+
+out:
+  return res;
+}
+
+/******************************************************************************/
+
+/* Prepend the header to the rlist */
+
+static hdr_rlist *
+arc_rlist_entry(hdr_rlist * list, const uschar * s, int len)
+{
+hdr_rlist * r = store_get(sizeof(hdr_rlist) + sizeof(header_line), FALSE);
+header_line * h = r->h = (header_line *)(r+1);
+
+r->prev = list;
+r->used = FALSE;
+h->next = NULL;
+h->type = 0;
+h->slen = len;
+h->text = US s;
+
+/* This works for either NL or CRLF lines; also nul-termination */
+while (*++s)
+  if (*s == '\n' && s[1] != '\t' && s[1] != ' ') break;
+s++;		/* move past end of line */
+
+return r;
+}
+
+
+/* Walk the given headers strings identifying each header, and construct
+a reverse-order list.
+*/
+
+static hdr_rlist *
+arc_sign_scan_headers(arc_ctx * ctx, gstring * sigheaders)
+{
+const uschar * s;
+hdr_rlist * rheaders = NULL;
+
+s = sigheaders ? sigheaders->s : NULL;
+if (s) while (*s)
+  {
+  const uschar * s2 = s;
+
+  /* This works for either NL or CRLF lines; also nul-termination */
+  while (*++s2)
+    if (*s2 == '\n' && s2[1] != '\t' && s2[1] != ' ') break;
+  s2++;		/* move past end of line */
+
+  rheaders = arc_rlist_entry(rheaders, s, s2 - s);
+  s = s2;
+  }
+return rheaders;
+}
+
+
+
+/* Return the A-R content, without identity, with line-ending and
+NUL termination. */
+
+static BOOL
+arc_sign_find_ar(header_line * headers, const uschar * identity, blob * ret)
+{
+header_line * h;
+int ilen = Ustrlen(identity);
+
+ret->data = NULL;
+for(h = headers; h; h = h->next)
+  {
+  uschar * s = h->text, c;
+  int len = h->slen;
+
+  if (Ustrncmp(s, HDR_AR, HDRLEN_AR) != 0) continue;
+  s += HDRLEN_AR, len -= HDRLEN_AR;		/* header name */
+  while (  len > 0
+	&& (c = *s) && (c == ' ' || c == '\t' || c == '\r' || c == '\n'))
+    s++, len--;					/* FWS */
+  if (Ustrncmp(s, identity, ilen) != 0) continue;
+  s += ilen; len -= ilen;			/* identity */
+  if (len <= 0) continue;
+  if ((c = *s) && c == ';') s++, len--;		/* identity terminator */
+  while (  len > 0
+	&& (c = *s) && (c == ' ' || c == '\t' || c == '\r' || c == '\n'))
+    s++, len--;					/* FWS */
+  if (len <= 0) continue;
+  ret->data = s;
+  ret->len = len;
+  return TRUE;
+  }
+return FALSE;
+}
+
+
+
+/* Append a constructed AAR including CRLF.  Add it to the arc_ctx too.  */
+
+static gstring *
+arc_sign_append_aar(gstring * g, arc_ctx * ctx,
+  const uschar * identity, int instance, blob * ar)
+{
+int aar_off = gstring_length(g);
+arc_set * as =
+  store_get(sizeof(arc_set) + sizeof(arc_line) + sizeof(header_line), FALSE);
+arc_line * al = (arc_line *)(as+1);
+header_line * h = (header_line *)(al+1);
+
+g = string_catn(g, ARC_HDR_AAR, ARC_HDRLEN_AAR);
+g = string_fmt_append(g, " i=%d; %s;\r\n\t", instance, identity);
+g = string_catn(g, US ar->data, ar->len);
+
+h->slen = g->ptr - aar_off;
+h->text = g->s + aar_off;
+al->complete = h;
+as->next = NULL;
+as->prev = ctx->arcset_chain_last;
+as->instance = instance;
+as->hdr_aar = al;
+if (instance == 1)
+  ctx->arcset_chain = as;
+else
+  ctx->arcset_chain_last->next = as;
+ctx->arcset_chain_last = as;
+
+DEBUG(D_transport) debug_printf("ARC: AAR '%.*s'\n", h->slen - 2, h->text);
+return g;
+}
+
+
+
+static BOOL
+arc_sig_from_pseudoheader(gstring * hdata, int hashtype, const uschar * privkey,
+  blob * sig, const uschar * why)
+{
+hashmethod hm = /*sig->keytype == KEYTYPE_ED25519*/ FALSE
+  ? HASH_SHA2_512 : pdkim_hashes[hashtype].exim_hashmethod;
+blob hhash;
+es_ctx sctx;
+const uschar * errstr;
+
+DEBUG(D_transport)
+  {
+  hctx hhash_ctx;
+  debug_printf("ARC: %s header data for signing:\n", why);
+  pdkim_quoteprint(hdata->s, hdata->ptr);
+
+  (void) exim_sha_init(&hhash_ctx, pdkim_hashes[hashtype].exim_hashmethod);
+  exim_sha_update(&hhash_ctx, hdata->s, hdata->ptr);
+  exim_sha_finish(&hhash_ctx, &hhash);
+  debug_printf("ARC: header hash: "); pdkim_hexprint(hhash.data, hhash.len);
+  }
+
+if (FALSE /*need hash for Ed25519 or GCrypt signing*/ )
+  {
+  hctx hhash_ctx;
+  (void) exim_sha_init(&hhash_ctx, pdkim_hashes[hashtype].exim_hashmethod);
+  exim_sha_update(&hhash_ctx, hdata->s, hdata->ptr);
+  exim_sha_finish(&hhash_ctx, &hhash);
+  }
+else
+  {
+  hhash.data = hdata->s;
+  hhash.len = hdata->ptr;
+  }
+
+if (  (errstr = exim_dkim_signing_init(privkey, &sctx))
+   || (errstr = exim_dkim_sign(&sctx, hm, &hhash, sig)))
+  {
+  log_write(0, LOG_MAIN, "ARC: %s signing: %s\n", why, errstr);
+  DEBUG(D_transport)
+    debug_printf("private key, or private-key file content, was: '%s'\n",
+      privkey);
+  return FALSE;
+  }
+return TRUE;
+}
+
+
+
+static gstring *
+arc_sign_append_sig(gstring * g, blob * sig)
+{
+/*debug_printf("%s: raw sig ", __FUNCTION__); pdkim_hexprint(sig->data, sig->len);*/
+sig->data = pdkim_encode_base64(sig);
+sig->len = Ustrlen(sig->data);
+for (;;)
+  {
+  int len = MIN(sig->len, 74);
+  g = string_catn(g, sig->data, len);
+  if ((sig->len -= len) == 0) break;
+  sig->data += len;
+  g = string_catn(g, US"\r\n\t  ", 5);
+  }
+g = string_catn(g, US";\r\n", 3);
+gstring_release_unused(g);
+string_from_gstring(g);
+return g;
+}
+
+
+/* Append a constructed AMS including CRLF.  Add it to the arc_ctx too. */
+
+static gstring *
+arc_sign_append_ams(gstring * g, arc_ctx * ctx, int instance,
+  const uschar * identity, const uschar * selector, blob * bodyhash,
+  hdr_rlist * rheaders, const uschar * privkey, unsigned options)
+{
+uschar * s;
+gstring * hdata = NULL;
+int col;
+int hashtype = pdkim_hashname_to_hashtype(US"sha256", 6);	/*XXX hardwired */
+blob sig;
+int ams_off;
+arc_line * al = store_get(sizeof(header_line) + sizeof(arc_line), FALSE);
+header_line * h = (header_line *)(al+1);
+
+/* debug_printf("%s\n", __FUNCTION__); */
+
+/* Construct the to-be-signed AMS pseudo-header: everything but the sig. */
+
+ams_off = g->ptr;
+g = string_fmt_append(g, "%s i=%d; a=rsa-sha256; c=relaxed; d=%s; s=%s",
+      ARC_HDR_AMS, instance, identity, selector);	/*XXX hardwired a= */
+if (options & ARC_SIGN_OPT_TSTAMP)
+  g = string_fmt_append(g, "; t=%lu", (u_long)now);
+if (options & ARC_SIGN_OPT_EXPIRE)
+  g = string_fmt_append(g, "; x=%lu", (u_long)expire);
+g = string_fmt_append(g, ";\r\n\tbh=%s;\r\n\th=",
+      pdkim_encode_base64(bodyhash));
+
+for(col = 3; rheaders; rheaders = rheaders->prev)
+  {
+  const uschar * hnames = US"DKIM-Signature:" PDKIM_DEFAULT_SIGN_HEADERS;
+  uschar * name, * htext = rheaders->h->text;
+  int sep = ':';
+
+  /* Spot headers of interest */
+
+  while ((name = string_nextinlist(&hnames, &sep, NULL, 0)))
+    {
+    int len = Ustrlen(name);
+    if (strncasecmp(CCS htext, CCS name, len) == 0)
+      {
+      /* If too long, fold line in h= field */
+
+      if (col + len > 78) g = string_catn(g, US"\r\n\t  ", 5), col = 3;
+
+      /* Add name to h= list */
+
+      g = string_catn(g, name, len);
+      g = string_catn(g, US":", 1);
+      col += len + 1;
+
+      /* Accumulate header for hashing/signing */
+
+      hdata = string_cat(hdata,
+		pdkim_relax_header_n(htext, rheaders->h->slen, TRUE));	/*XXX hardwired */
+      break;
+      }
+    }
+  }
+
+/* Lose the last colon from the h= list */
+
+if (g->s[g->ptr - 1] == ':') g->ptr--;
+
+g = string_catn(g, US";\r\n\tb=;", 7);
+
+/* Include the pseudo-header in the accumulation */
+
+s = pdkim_relax_header_n(g->s + ams_off, g->ptr - ams_off, FALSE);
+hdata = string_cat(hdata, s);
+
+/* Calculate the signature from the accumulation */
+/*XXX does that need further relaxation? there are spaces embedded in the b= strings! */
+
+if (!arc_sig_from_pseudoheader(hdata, hashtype, privkey, &sig, US"AMS"))
+  return NULL;
+
+/* Lose the trailing semicolon from the psuedo-header, and append the signature
+(folded over lines) and termination to complete it. */
+
+g->ptr--;
+g = arc_sign_append_sig(g, &sig);
+
+h->slen = g->ptr - ams_off;
+h->text = g->s + ams_off;
+al->complete = h;
+ctx->arcset_chain_last->hdr_ams = al;
+
+DEBUG(D_transport) debug_printf("ARC: AMS '%.*s'\n", h->slen - 2, h->text);
+return g;
+}
+
+
+
+/* Look for an arc= result in an A-R header blob.  We know that its data
+happens to be a NUL-term string. */
+
+static uschar *
+arc_ar_cv_status(blob * ar)
+{
+const uschar * resinfo = ar->data;
+int sep = ';';
+uschar * methodspec, * s;
+
+while ((methodspec = string_nextinlist(&resinfo, &sep, NULL, 0)))
+  if (Ustrncmp(methodspec, US"arc=", 4) == 0)
+    {
+    uschar c;
+    for (s = methodspec += 4;
+         (c = *s) && c != ';' && c != ' ' && c != '\r' && c != '\n'; ) s++;
+    return string_copyn(methodspec, s - methodspec);
+    }
+return US"none";
+}
+
+
+
+/* Build the AS header and prepend it */
+
+static gstring *
+arc_sign_prepend_as(gstring * arcset_interim, arc_ctx * ctx,
+  int instance, const uschar * identity, const uschar * selector, blob * ar,
+  const uschar * privkey, unsigned options)
+{
+gstring * arcset;
+uschar * status = arc_ar_cv_status(ar);
+arc_line * al = store_get(sizeof(header_line) + sizeof(arc_line), FALSE);
+header_line * h = (header_line *)(al+1);
+uschar * badline_str;
+
+gstring * hdata = NULL;
+int hashtype = pdkim_hashname_to_hashtype(US"sha256", 6);	/*XXX hardwired */
+blob sig;
+
+/*
+- Generate AS
+  - no body coverage
+  - no h= tag; implicit coverage
+  - arc status from A-R
+    - if fail:
+      - coverage is just the new ARC set
+        including self (but with an empty b= in self)
+    - if non-fail:
+      - all ARC set headers, set-number order, aar then ams then as,
+        including self (but with an empty b= in self)
+*/
+DEBUG(D_transport) debug_printf("ARC: building AS for status '%s'\n", status);
+
+/* Construct the AS except for the signature */
+
+arcset = string_append(NULL, 9,
+	  ARC_HDR_AS,
+	  US" i=", string_sprintf("%d", instance),
+	  US"; cv=", status,
+	  US"; a=rsa-sha256; d=", identity,			/*XXX hardwired */
+	  US"; s=", selector);					/*XXX same as AMS */
+if (options & ARC_SIGN_OPT_TSTAMP)
+  arcset = string_append(arcset, 2,
+      US"; t=", string_sprintf("%lu", (u_long)now));
+arcset = string_cat(arcset,
+	  US";\r\n\t b=;");
+
+h->slen = arcset->ptr;
+h->text = arcset->s;
+al->complete = h;
+ctx->arcset_chain_last->hdr_as = al;
+
+/* For any but "fail" chain-verify status, walk the entire chain in order by
+instance.  For fail, only the new arc-set.  Accumulate the elements walked. */
+
+for (arc_set * as = Ustrcmp(status, US"fail") == 0
+	? ctx->arcset_chain_last : ctx->arcset_chain;
+     as; as = as->next)
+  {
+  arc_line * l;
+  /* Accumulate AAR then AMS then AS.  Relaxed canonicalisation
+  is required per standard. */
+
+  badline_str = US"aar";
+  if (!(l = as->hdr_aar)) goto badline;
+  h = l->complete;
+  hdata = string_cat(hdata, pdkim_relax_header_n(h->text, h->slen, TRUE));
+  badline_str = US"ams";
+  if (!(l = as->hdr_ams)) goto badline;
+  h = l->complete;
+  hdata = string_cat(hdata, pdkim_relax_header_n(h->text, h->slen, TRUE));
+  badline_str = US"as";
+  if (!(l = as->hdr_as)) goto badline;
+  h = l->complete;
+  hdata = string_cat(hdata, pdkim_relax_header_n(h->text, h->slen, !!as->next));
+  }
+
+/* Calculate the signature from the accumulation */
+
+if (!arc_sig_from_pseudoheader(hdata, hashtype, privkey, &sig, US"AS"))
+  return NULL;
+
+/* Lose the trailing semicolon */
+arcset->ptr--;
+arcset = arc_sign_append_sig(arcset, &sig);
+DEBUG(D_transport) debug_printf("ARC: AS  '%.*s'\n", arcset->ptr - 2, arcset->s);
+
+/* Finally, append the AMS and AAR to the new AS */
+
+return string_catn(arcset, arcset_interim->s, arcset_interim->ptr);
+
+badline:
+  DEBUG(D_transport)
+    debug_printf("ARC: while building AS, missing %s in chain\n", badline_str);
+  return NULL;
+}
+
+
+/**************************************/
+
+/* Return pointer to pdkim_bodyhash for given hash method, creating new
+method if needed.
+*/
+
+void *
+arc_ams_setup_sign_bodyhash(void)
+{
+int canon_head, canon_body;
+
+DEBUG(D_transport) debug_printf("ARC: requesting bodyhash\n");
+pdkim_cstring_to_canons(US"relaxed", 7, &canon_head, &canon_body);	/*XXX hardwired */
+return pdkim_set_bodyhash(&dkim_sign_ctx,
+	pdkim_hashname_to_hashtype(US"sha256", 6),			/*XXX hardwired */
+	canon_body,
+	-1);
+}
+
+
+
+void
+arc_sign_init(void)
+{
+memset(&arc_sign_ctx, 0, sizeof(arc_sign_ctx));
+}
+
+
+
+/* A "normal" header line, identified by DKIM processing.  These arrive before
+the call to arc_sign(), which carries any newly-created DKIM headers - and
+those go textually before the normal ones in the message.
+
+We have to take the feed from DKIM as, in the transport-filter case, the
+headers are not in memory at the time of the call to arc_sign().
+
+Take a copy of the header and construct a reverse-order list.
+Also parse ARC-chain headers and build the chain struct, retaining pointers
+into the copies.
+*/
+
+static const uschar *
+arc_header_sign_feed(gstring * g)
+{
+uschar * s = string_copyn(g->s, g->ptr);
+headers_rlist = arc_rlist_entry(headers_rlist, s, g->ptr);
+return arc_try_header(&arc_sign_ctx, headers_rlist->h, TRUE);
+}
+
+
+
+/* ARC signing.  Called from the smtp transport, if the arc_sign option is set.
+The dkim_exim_sign() function has already been called, so will have hashed the
+message body for us so long as we requested a hash previously.
+
+Arguments:
+  signspec	Three-element colon-sep list: identity, selector, privkey.
+		Optional fourth element: comma-sep list of options.
+		Already expanded
+  sigheaders	Any signature headers already generated, eg. by DKIM, or NULL
+  errstr	Error string
+
+Return value
+  Set of headers to prepend to the message, including the supplied sigheaders
+  but not the plainheaders.
+*/
+
+gstring *
+arc_sign(const uschar * signspec, gstring * sigheaders, uschar ** errstr)
+{
+const uschar * identity, * selector, * privkey, * opts, * s;
+unsigned options = 0;
+int sep = 0;
+header_line * headers;
+hdr_rlist * rheaders;
+blob ar;
+int instance;
+gstring * g = NULL;
+pdkim_bodyhash * b;
+
+expire = now = 0;
+
+/* Parse the signing specification */
+
+identity = string_nextinlist(&signspec, &sep, NULL, 0);
+selector = string_nextinlist(&signspec, &sep, NULL, 0);
+if (  !*identity || !*selector
+   || !(privkey = string_nextinlist(&signspec, &sep, NULL, 0)) || !*privkey)
+  {
+  log_write(0, LOG_MAIN, "ARC: bad signing-specification (%s)",
+    !*identity ? "identity" : !*selector ? "selector" : "private-key");
+  return sigheaders ? sigheaders : string_get(0);
+  }
+if (*privkey == '/' && !(privkey = expand_file_big_buffer(privkey)))
+  return sigheaders ? sigheaders : string_get(0);
+
+if ((opts = string_nextinlist(&signspec, &sep, NULL, 0)))
+  {
+  int osep = ',';
+  while ((s = string_nextinlist(&opts, &osep, NULL, 0)))
+    if (Ustrcmp(s, "timestamps") == 0)
+      {
+      options |= ARC_SIGN_OPT_TSTAMP;
+      if (!now) now = time(NULL);
+      }
+    else if (Ustrncmp(s, "expire", 6) == 0)
+      {
+      options |= ARC_SIGN_OPT_EXPIRE;
+      if (*(s += 6) == '=')
+	if (*++s == '+')
+	  {
+	  if (!(expire = (time_t)atoi(CS ++s)))
+	    expire = ARC_SIGN_DEFAULT_EXPIRE_DELTA;
+	  if (!now) now = time(NULL);
+	  expire += now;
+	  }
+	else
+	  expire = (time_t)atol(CS s);
+      else
+	{
+	if (!now) now = time(NULL);
+	expire = now + ARC_SIGN_DEFAULT_EXPIRE_DELTA;
+	}
+      }
+  }
+
+DEBUG(D_transport) debug_printf("ARC: sign for %s\n", identity);
+
+/* Make an rlist of any new DKIM headers, then add the "normals" rlist to it.
+Then scan the list for an A-R header. */
+
+string_from_gstring(sigheaders);
+if ((rheaders = arc_sign_scan_headers(&arc_sign_ctx, sigheaders)))
+  {
+  hdr_rlist ** rp;
+  for (rp = &headers_rlist; *rp; ) rp = &(*rp)->prev;
+  *rp = rheaders;
+  }
+
+/* Finally, build a normal-order headers list */
+/*XXX only needed for hunt-the-AR? */
+/*XXX also, we really should be accepting any number of ADMD-matching ARs */
+  {
+  header_line * hnext = NULL;
+  for (rheaders = headers_rlist; rheaders;
+       hnext = rheaders->h, rheaders = rheaders->prev)
+    rheaders->h->next = hnext;
+  headers = hnext;
+  }
+
+if (!(arc_sign_find_ar(headers, identity, &ar)))
+  {
+  log_write(0, LOG_MAIN, "ARC: no Authentication-Results header for signing");
+  return sigheaders ? sigheaders : string_get(0);
+  }
+
+/* We previously built the data-struct for the existing ARC chain, if any, using a headers
+feed from the DKIM module.  Use that to give the instance number for the ARC set we are
+about to build. */
+
+DEBUG(D_transport)
+  if (arc_sign_ctx.arcset_chain_last)
+    debug_printf("ARC: existing chain highest instance: %d\n",
+      arc_sign_ctx.arcset_chain_last->instance);
+  else
+    debug_printf("ARC: no existing chain\n");
+
+instance = arc_sign_ctx.arcset_chain_last ? arc_sign_ctx.arcset_chain_last->instance + 1 : 1;
+
+/*
+- Generate AAR
+  - copy the A-R; prepend i= & identity
+*/
+
+g = arc_sign_append_aar(g, &arc_sign_ctx, identity, instance, &ar);
+
+/*
+- Generate AMS
+  - Looks fairly like a DKIM sig
+  - Cover all DKIM sig headers as well as the usuals
+    - ? oversigning?
+  - Covers the data
+  - we must have requested a suitable bodyhash previously
+*/
+
+b = arc_ams_setup_sign_bodyhash();
+g = arc_sign_append_ams(g, &arc_sign_ctx, instance, identity, selector,
+      &b->bh, headers_rlist, privkey, options);
+
+/*
+- Generate AS
+  - no body coverage
+  - no h= tag; implicit coverage
+  - arc status from A-R
+    - if fail:
+      - coverage is just the new ARC set
+        including self (but with an empty b= in self)
+    - if non-fail:
+      - all ARC set headers, set-number order, aar then ams then as,
+        including self (but with an empty b= in self)
+*/
+
+if (g)
+  g = arc_sign_prepend_as(g, &arc_sign_ctx, instance, identity, selector, &ar,
+      privkey, options);
+
+/* Finally, append the dkim headers and return the lot. */
+
+if (sigheaders) g = string_catn(g, sigheaders->s, sigheaders->ptr);
+(void) string_from_gstring(g);
+gstring_release_unused(g);
+return g;
+}
+
+
+/******************************************************************************/
+
+/* Check to see if the line is an AMS and if so, set up to validate it.
+Called from the DKIM input processing.  This must be done now as the message
+body data is hashed during input.
+
+We call the DKIM code to request a body-hash; it has the facility already
+and the hash parameters might be common with other requests.
+*/
+
+static const uschar *
+arc_header_vfy_feed(gstring * g)
+{
+header_line h;
+arc_line al;
+pdkim_bodyhash * b;
+uschar * errstr;
+
+if (!dkim_verify_ctx) return US"no dkim context";
+
+if (strncmpic(ARC_HDR_AMS, g->s, ARC_HDRLEN_AMS) != 0) return US"not AMS";
+
+DEBUG(D_receive) debug_printf("ARC: spotted AMS header\n");
+/* Parse the AMS header */
+
+h.next = NULL;
+h.slen = g->size;
+h.text = g->s;
+memset(&al, 0, sizeof(arc_line));
+if ((errstr = arc_parse_line(&al, &h, ARC_HDRLEN_AMS, FALSE)))
+  {
+  DEBUG(D_acl) if (errstr) debug_printf("ARC: %s\n", errstr);
+  goto badline;
+  }
+
+if (!al.a_hash.data)
+  {
+  DEBUG(D_acl) debug_printf("ARC: no a_hash from '%.*s'\n", h.slen, h.text);
+  goto badline;
+  }
+
+/* defaults */
+if (!al.c.data)
+  {
+  al.c_body.data = US"simple"; al.c_body.len = 6;
+  al.c_head = al.c_body;
+  }
+
+/* Ask the dkim code to calc a bodyhash with those specs */
+
+if (!(b = arc_ams_setup_vfy_bodyhash(&al)))
+  return US"dkim hash setup fail";
+
+/* Discard the reference; search again at verify time, knowing that one
+should have been created here. */
+
+return NULL;
+
+badline:
+  return US"line parsing error";
+}
+
+
+
+/* A header line has been identified by DKIM processing.
+
+Arguments:
+  g		Header line
+  is_vfy	TRUE for verify mode or FALSE for signing mode
+
+Return:
+  NULL for success, or an error string (probably unused)
+*/
+
+const uschar *
+arc_header_feed(gstring * g, BOOL is_vfy)
+{
+return is_vfy ? arc_header_vfy_feed(g) : arc_header_sign_feed(g);
+}
+
+
+
+/******************************************************************************/
+
+/* Construct the list of domains from the ARC chain after validation */
+
+uschar *
+fn_arc_domains(void)
+{
+arc_set * as;
+unsigned inst;
+gstring * g = NULL;
+
+for (as = arc_verify_ctx.arcset_chain, inst = 1; as; as = as->next, inst++)
+  {
+  arc_line * hdr_as = as->hdr_as;
+  if (hdr_as)
+    {
+    blob * d = &hdr_as->d;
+
+    for (; inst < as->instance; inst++)
+      g = string_catn(g, US":", 1);
+
+    g = d->data && d->len
+      ? string_append_listele_n(g, ':', d->data, d->len)
+      : string_catn(g, US":", 1);
+    }
+  else
+    g = string_catn(g, US":", 1);
+  }
+return g ? g->s : US"";
+}
+
+
+/* Construct an Authentication-Results header portion, for the ARC module */
+
+gstring *
+authres_arc(gstring * g)
+{
+if (arc_state)
+  {
+  arc_line * highest_ams;
+  int start = 0;		/* Compiler quietening */
+  DEBUG(D_acl) start = g->ptr;
+
+  g = string_append(g, 2, US";\n\tarc=", arc_state);
+  if (arc_received_instance > 0)
+    {
+    g = string_fmt_append(g, " (i=%d)", arc_received_instance);
+    if (arc_state_reason)
+      g = string_append(g, 3, US"(", arc_state_reason, US")");
+    g = string_catn(g, US" header.s=", 10);
+    highest_ams = arc_received->hdr_ams;
+    g = string_catn(g, highest_ams->s.data, highest_ams->s.len);
+
+    g = string_fmt_append(g, " arc.oldest-pass=%d", arc_oldest_pass);
+
+    if (sender_host_address)
+      g = string_append(g, 2, US" smtp.remote-ip=", sender_host_address);
+    }
+  else if (arc_state_reason)
+    g = string_append(g, 3, US" (", arc_state_reason, US")");
+  DEBUG(D_acl) debug_printf("ARC:  authres '%.*s'\n",
+		  g->ptr - start - 3, g->s + start + 3);
+  }
+else
+  DEBUG(D_acl) debug_printf("ARC:  no authres\n");
+return g;
+}
+
+
+# endif /* DISABLE_DKIM */
+#endif /* EXPERIMENTAL_ARC */
+/* vi: aw ai sw=2
+ */