c2e25edbf8c48361a71398664ec0c279d5fc58f5
braney
  Sat Sep 19 18:11:31 2026 -0700
hmacTest: pin the signature hgLogin puts on a pending social identity

hgLogin signs a pending social identity with hmacMd5(login.cookieSalt, fields)
and hands the signature to the browser in the account chooser, so a forgeable
signature lets an attacker choose whose account the chooser links to.  Nothing
about the page looks different either way.

Before #37984 that signature was a plain MD5 of the salt concatenated in front
of the same fields, and a hash of a secret followed by attacker-chosen text is
the wrong shape for the job.  boundaryMoves() is the case that says so: with
concatenation, hmacMd5("a", "bc") and hmacMd5("ab", "c") hash the same bytes
and sign the same, while HMAC keeps them apart.

Known answers are RFC 2202 test case 2 for MD5 and SHA1, cross-checked here
with the openssl command line.  Only the string-keyed vectors from the RFC can
be used, since this interface takes key and data as C strings.

Watched to fail and then pass: putting hmacMd5 back to the concatenation shape
turns the known answer red and makes the two boundary cases identical, both of
which the test catches.  Recorded as sandbox-ab in utils/testRegistry.

refs #37984, refs #38391

diff --git src/lib/tests/hmacTest.c src/lib/tests/hmacTest.c
new file mode 100644
index 00000000000..4f9c0797ef5
--- /dev/null
+++ src/lib/tests/hmacTest.c
@@ -0,0 +1,134 @@
+/* hmacTest - check hmacMd5 and hmacSha1 against known answers, and check the one
+ * property hgLogin depends on.
+ *
+ * hgLogin signs a pending social identity with hmacMd5(login.cookieSalt, "provider|id|email")
+ * and hands the signature to the browser in the account chooser.  If the signature can be
+ * forged, an attacker picks whose account the chooser links to, so this is a security
+ * boundary with no visible symptom: a forged signature and a real one produce the same page.
+ *
+ * Before #37984 that signature was a plain MD5 of the salt concatenated in front of the same
+ * fields.  A plain hash of a secret followed by attacker-chosen text is the wrong shape for
+ * the job, and the test that says so is boundaryMoves() below: with concatenation, moving a
+ * character from the end of the key to the front of the data leaves the hashed bytes
+ * identical, so two different (key, data) pairs sign the same.  HMAC keeps them apart, and a
+ * change that quietly went back to concatenation would fail there rather than in a page
+ * nobody can tell apart.
+ *
+ * The known answers are RFC 2202 test case 2 for both algorithms, cross-checked on this
+ * machine with `openssl dgst -md5 -hmac Jefe`.  Only the string-keyed cases from the RFC are
+ * usable here, because this interface takes the key and the data as C strings and measures
+ * both with strlen, so a vector with an embedded zero or a 0x0b key cannot be expressed.
+ *
+ * refs #37984 */
+
+/* Copyright (C) 2026 The Regents of the University of California
+ * See kent/LICENSE or http://genome.ucsc.edu/license/ for licensing information. */
+
+#include "common.h"
+#include "hmac.h"
+
+static int errCount = 0;
+
+static void expect(char *what, char *got, char *want)
+/* Print the answer, and say so when it is not the one expected.  Printing every line rather
+ * than only the failures keeps the expected/ file a readable record of what this pins. */
+{
+printf("%-34s %s\n", what, got);
+if (!sameString(got, want))
+    {
+    printf("    FAIL: expected %s\n", want);
+    ++errCount;
+    }
+}
+
+static void knownAnswers()
+/* RFC 2202 test case 2, the one whose key and data are both plain text. */
+{
+char *key = "Jefe";
+char *data = "what do ya want for nothing?";
+expect("md5  rfc2202 case 2", hmacMd5(key, data),
+       "750c783e6ab0b503eaa86e310a5db738");
+expect("sha1 rfc2202 case 2", hmacSha1(key, data),
+       "effcdf6ae5eb2fa2d27416d5f184df9c259a7c79");
+}
+
+static void boundaryMoves()
+/* The property hgLogin's signature rests on: the split between key and data is part of what
+ * is signed.  Concatenating the two, which is what the code did before #37984, makes these
+ * two calls hash the same bytes. */
+{
+char *ab = hmacMd5("a", "bc");
+char *aB = hmacMd5("ab", "c");
+printf("%-34s %s\n", "md5  key a  data bc", ab);
+printf("%-34s %s\n", "md5  key ab data c", aB);
+if (sameString(ab, aB))
+    {
+    printf("    FAIL: the key/data boundary is not being signed\n");
+    ++errCount;
+    }
+else
+    printf("%-34s differ, as they must\n", "    the two");
+}
+
+static void keyMatters()
+/* A signature that does not depend on the whole key is not a signature.  One bit of the salt
+ * has to change the answer. */
+{
+char *withSalt = hmacMd5("s3cret", "google|12345|a@example.org");
+char *withOther = hmacMd5("s3crat", "google|12345|a@example.org");
+printf("%-34s %s\n", "md5  salt s3cret", withSalt);
+printf("%-34s %s\n", "md5  salt s3crat", withOther);
+if (sameString(withSalt, withOther))
+    {
+    printf("    FAIL: the key does not reach the answer\n");
+    ++errCount;
+    }
+}
+
+static void shapeOfTheAnswer()
+/* hgLogin puts the signature in a form field and compares it with sameString, so the text has
+ * to be exactly this: lower case hex, no prefix, and the full width of the digest.  The
+ * buffers in hmac.c are sized for exactly these lengths. */
+{
+char *md5 = hmacMd5("k", "d");
+char *sha1 = hmacSha1("k", "d");
+printf("%-34s %d\n", "md5  length", (int)strlen(md5));
+printf("%-34s %d\n", "sha1 length", (int)strlen(sha1));
+if (strlen(md5) != 32 || strlen(sha1) != 40)
+    {
+    printf("    FAIL: expected 32 and 40\n");
+    ++errCount;
+    }
+int i;
+for (i = 0;  md5[i] != '\0';  ++i)
+    if (strchr("0123456789abcdef", md5[i]) == NULL)
+        {
+        printf("    FAIL: '%c' is not lower case hex\n", md5[i]);
+        ++errCount;
+        break;
+        }
+}
+
+static void emptyData()
+/* An empty string is a legitimate thing to sign, and it must not come back empty or equal to
+ * the signature of anything else. */
+{
+char *empty = hmacMd5("k", "");
+printf("%-34s %s\n", "md5  empty data", empty);
+if (strlen(empty) != 32 || sameString(empty, hmacMd5("k", "x")))
+    {
+    printf("    FAIL: empty data signs wrong\n");
+    ++errCount;
+    }
+}
+
+int main(int argc, char *argv[])
+{
+knownAnswers();
+boundaryMoves();
+keyMatters();
+shapeOfTheAnswer();
+emptyData();
+printf("\n%d failures\n", errCount);
+return errCount == 0 ? 0 : 1;
+}