diff --git a/lilcrypto.h b/lilcrypto.h
index f599c36..3e2fdb2 100644
--- a/lilcrypto.h
+++ b/lilcrypto.h
@@ -16,6 +16,7 @@
 
 #include <stddef.h>
 #include <stdint.h>
+#include <stdio.h>
 
 
 /*
@@ -110,4 +111,5 @@ const struct lc_aead_impl	*lc_aead_impl_chacha20_poly1305(void);
  * Utilities.
  */
 
+int	lc_hexdump_fp(FILE *, const void *, size_t);
 void	lc_scrub(void *, size_t);
diff --git a/util.c b/util.c
index 9836368..5e38290 100644
--- a/util.c
+++ b/util.c
@@ -14,11 +14,107 @@
  * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  */
 
+#include <ctype.h>
 #include <string.h>
 
 #include "lilcrypto.h"
 
 
+#define HEXDUMP_BUFSZ	128
+
+
+static size_t
+hexdump_line(char *buf, const uint8_t *blob, size_t len, size_t off, int pad)
+{
+	/*
+	 * Format is
+	 * - 16-char hex offset (at most) 
+	 * - 2 spaces
+	 * - 2-char hex byte and a space (16 times)
+	 * - 2 spaces
+	 * - 2 pipe chars + 16 renders of the blobs
+	 *
+	 * That accounts for, at most, 16 + 2 + 16 * 3 + 2 + 18. Adding an
+	 * extra byte for the NUL ending byte, that amounts for 87.
+	 *
+	 * Callers MUST provide a buffer at least HEXDUMP_BUFSZ long in buf0.
+	 */
+
+	char	*bufp;
+	size_t	 i, buflen;
+	int	 w;
+
+	bufp = buf;
+	buflen = HEXDUMP_BUFSZ;
+
+	if (len == 0)
+		(void)snprintf(bufp, buflen, "%0*zx\n", pad, off);
+	else {
+		w = snprintf(bufp, buflen, "%0*zx  ", pad, off);
+		bufp += w;
+		buflen -= w;
+		for (i = 0; i < len && i < 8; i++) {
+			w = snprintf(bufp, buflen, "%02x ", blob[i]);
+			bufp += w;
+			buflen -= w;
+		}
+		if (i < 8) {
+			memset(bufp, ' ', 3 * (16 - i));
+			bufp += 3 * (16 - i);
+			buflen -= 3 * (16 - i);
+		}
+		for (; i < len && i < 16; i++) {
+			w = snprintf(bufp, buflen, " %02x", blob[i]);
+			bufp += w;
+			buflen -= w;
+		}
+		if (i < 16) {
+			memset(bufp, ' ', 3 * (16 - i));
+			bufp += 3 * (16 - i);
+			buflen -= 3 * (16 - i);
+		}
+		w = snprintf(bufp, buflen, "  |");
+		bufp += w;
+		buflen -= w;
+		for (i = 0; i < len && i < 16; i++) {
+			w = snprintf(bufp, buflen, "%c",
+			    isprint(blob[i]) ? blob[i] : '.');
+			bufp += w;
+			buflen -= w;
+		}
+		(void)snprintf(bufp, buflen, "|\n");
+	}
+
+	return len < 16 ? len : 16;
+}
+
+int
+lc_hexdump_fp(FILE *fp, const void *blob, size_t len)
+{
+	const uint8_t	*p = blob;
+	char		 buf[HEXDUMP_BUFSZ];
+	size_t		 l, off;
+	int		 pad;
+
+	for (pad = 1; len > (1 << (8 * pad)); pad++)
+		continue;
+	pad *= 2;
+
+	off = 0;
+	while (len > 0) {
+		l = hexdump_line(buf, p, len, off, pad);
+		if (fputs(buf, fp) == EOF)
+			return 0;
+		off += l;
+		len -= l;
+	}
+	(void)hexdump_line(buf, p, len, off, pad);
+	if (fputs(buf, fp) == EOF)
+		return 0;
+
+	return 1;
+}
+
 void
 lc_scrub(void *b, size_t len)
 {
diff --git a/wycheproof_aead.c b/wycheproof_aead.c
index 4955696..2d6f3b6 100644
--- a/wycheproof_aead.c
+++ b/wycheproof_aead.c
@@ -60,58 +60,6 @@ hexparse(const char *s, uint8_t *out, size_t *outlen)
 	return 1;
 }
 
-static void
-hexdump(FILE *fp, const uint8_t *blob, size_t len)
-{
-	size_t	i, off;
-	int	pad;
-
-	for (i = 0; len > (1 << (8 * i)); i++)
-		;
-	pad = (i + 1) * 2;
-
-	off = 0;
-	while (len >= 16) {
-		fprintf(fp, "%0*zx\t", pad, off);
-
-		for (i = 0; i < 8; i++)
-			fprintf(fp, "%02x ", blob[i]);
-		for (; i < 16; i++)
-			fprintf(fp, " %02x", blob[i]);
-
-		fprintf(fp, "\t|");
-		for (i = 0; i < 16; i++)
-			fprintf(fp, "%c", isprint(blob[i]) ? blob[i] : '.');
-		fprintf(fp, "|\n");
-
-		blob += 16;
-		off += 16;
-		len -= 16;
-	}
-
-	if (len == 0)
-		goto out;
-
-	fprintf(fp, "%0*zx\t", pad, off);
-	for (i = 0; i < len && i < 8; i++)
-		fprintf(fp, "%02x ", blob[i]);
-	for (; i < 8; i++)
-		fprintf(fp, "   ");
-	for (; i < len && i < 16; i++)
-		fprintf(fp, " %02x", blob[i]);
-	for (; i < 16; i++)
-		fprintf(fp, "   ");
-
-	fprintf(fp, "\t|");
-	for (i = 0; i < len; i++)
-		fprintf(fp, "%c", isprint(blob[i]) ? blob[i] : '.');
-	fprintf(fp, "|\n");
-
- out:
-	fprintf(fp, "%0*zx\n", pad, off + len);
-	fflush(fp);
-}
-
 struct kwimpl {
 	const char			*kw;
 	const struct lc_aead_impl	*(*impl)(void);
@@ -305,11 +253,11 @@ main(int argc, char *argv[])
 		if (verbose) {
 			fprintf(stderr, "ct (%zu, %zu)\n", ctlen,
 			    encoutlen - LC_POLY1305_TAGLEN);
-			hexdump(stderr, msg, msglen);
+			lc_hexdump_fp(stderr, msg, msglen);
 			fprintf(stderr, "\n");
-			hexdump(stderr, ct, ctlen);
+			lc_hexdump_fp(stderr, ct, ctlen);
 			fprintf(stderr, "\n");
-			hexdump(stderr, encout,
+			lc_hexdump_fp(stderr, encout,
 			    encoutlen - LC_POLY1305_TAGLEN);
 			fprintf(stderr, "\n");
 		}
@@ -321,9 +269,9 @@ main(int argc, char *argv[])
 		if (verbose) {
 			fprintf(stderr, "tag (%zu, %zu)\n", taglenarg,
 			    (size_t)LC_POLY1305_TAGLEN);
-			hexdump(stderr, tag, taglen);
+			lc_hexdump_fp(stderr, tag, taglen);
 			fprintf(stderr, "\n");
-			hexdump(stderr, encout + ctlen, LC_POLY1305_TAGLEN);
+			lc_hexdump_fp(stderr, encout + ctlen, LC_POLY1305_TAGLEN);
 			fprintf(stderr, "\n");
 		}
 		puts("invalid");
@@ -355,11 +303,11 @@ main(int argc, char *argv[])
 	if (msglen != decoutlen || lc_ct_cmp(decout, msg, msglen) != 0) {
 		if (verbose) {
 			fprintf(stderr, "ct (%zu, %zu)\n", msglen, decoutlen);
-			hexdump(stderr, msg, msglen);
+			lc_hexdump_fp(stderr, msg, msglen);
 			fprintf(stderr, "\n");
-			hexdump(stderr, ct, ctlen);
+			lc_hexdump_fp(stderr, ct, ctlen);
 			fprintf(stderr, "\n");
-			hexdump(stderr, decout, decoutlen);
+			lc_hexdump_fp(stderr, decout, decoutlen);
 			fprintf(stderr, "\n");
 		}
 		puts("invalid");