diff --git a/lisp/gnus/gnus-util.el b/lisp/gnus/gnus-util.el
index aa9f137e20..7e8dcaa47c 100644
--- a/lisp/gnus/gnus-util.el
+++ b/lisp/gnus/gnus-util.el
@@ -1343,6 +1343,58 @@ gnus-url-unhex-string
     (setq tmp (concat tmp str))
     tmp))
 
+(defun gnus-base64-repad (str &optional reject-newlines line-length)
+  "Take a base 64-encoded string and return it padded correctly,
+regardless of what padding the input string alread had.
+
+If any combination of CR and LF characters are present and
+REJECT-NEWLINES is nil remove them, otherwise raise an error. If
+LINE-LENGTH is set and the string (or any line in the string if
+REJECT-NEWLINES is nil) is longer than that number, raise an
+error. Common line length for input characters are 76 plus CRLF
+(RFC 2045 MIME), 64 plus CRLF (RFC 1421 PEM), and 1000 including
+CRLF (RFC 5321 SMTP).
+"
+  ;;; RFC 4648 specifies that:
+  ;;; - three 8-bit inputs make up a 24-bit group
+  ;;; - the 24-bit group is broken up into four 6-bit values
+  ;;; - each 6-bit value is mapped to one character of the base 64 alphabet
+  ;;; - if the final 24-bit quantum is filled with only 8 bits the output
+  ;;;   will be two base 64 characters followed by two "=" padding characters
+  ;;; - if the final 24-bit quantum is filled with only 16 bits the output
+  ;;;   will be three base 64 character followed by one "=" padding character
+  ;;;
+  ;;; RFC 4648 section 3 considerations:
+  ;;; - if reject-newlines is nil (default), concatenate multi-line
+  ;;;   input (3.1, 3.3)
+  ;;; - if line-length is set, error on input exceeding the limit (3.1)
+  ;;; - reject characters outside base encoding (3.3, also section 12)
+
+  (let ((splitstr (split-string str "[\r\n]" t)))
+    (progn
+      (if (and reject-newlines (> (length splitstr) 1))
+	  	 (error "Invalid Base64 string"))
+      (dolist (substr splitstr)
+	(if (and line-length (> (length substr) line-length))
+	    (error "Base64 string exceeds line-length"))
+	(if (string-match "[^A-Za-z0-9+/=]" substr)
+	    (error "Invalid Base64 string")))
+      (let* ((str (string-join splitstr))
+	     (len (length str))
+	     (padding nil))
+	(if (string-match "=" str)
+	    (setq len (match-beginning 0)))
+	(progn (setq padding
+		     (/
+		      (- 24
+			 (pcase (mod (* len 6) 24)
+			   (`0 24)
+			   (n n)))
+		      6))
+	       (concat
+		(substring str 0 len)
+		(make-string padding ?=)))))))
+
 (defun gnus-make-predicate (spec)
   "Transform SPEC into a function that can be called.
 SPEC is a predicate specifier that contains stuff like `or', `and',