diff --git a/lib/chibi/crypto/sha2.scm b/lib/chibi/crypto/sha2.scm
index 2378b0e6..fa5af55f 100644
--- a/lib/chibi/crypto/sha2.scm
+++ b/lib/chibi/crypto/sha2.scm
@@ -40,7 +40,11 @@
    (u32 (arithmetic-shift n (- 32 k)))
    (arithmetic-shift n (- k))))
 
-(define hex integer->hex-string)
+(define (hex32 num)
+  (let ((res (make-string 8 #\0))
+        (num (integer->hex-string num)))
+    (string-copy! res (- 8 (string-length num)) num)
+    res))
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
@@ -152,8 +156,8 @@
                 (if (>= n 56)
                     (chunk (+ i n) 0 a b c d e f g h)
                     (string-append
-                     (hex a) (hex b) (hex c) (hex d)
-                     (hex e) (hex f) (hex g) (if full? (hex h) ""))))
+                     (hex32 a) (hex32 b) (hex32 c) (hex32 d)
+                     (hex32 e) (hex32 f) (hex32 g) (if full? (hex32 h) ""))))
                (else
                 (chunk (+ i 64) pad a b c d e f g h)))))
            (else