openssl x509 (v1.0.1f) displays public key moduli as arrays of hex-encoded bytes, 15 columns wide, starting with a leading 00::

    Subject Public Key Info:
        Public Key Algorithm: rsaEncryption
            Public-Key: (1024 bit)
            Exponent: 65537 (0x10001)

GnuTLS's certtool -i shows the bytes in an array of 16 columns, which seems less surprising, but again starting with a leading 00::

Subject Public Key Algorithm: RSA
Certificate Security Level: Weak
    Modulus (bits 1024):
    Exponent (bits 24):

When it comes to private keys, however, both openssl rsa and certtool -i -k show all the private key components in arrays of bytes 15 columns wide. They both show leading 00: for some components of the private keys but not others.

This display format seems unusual because the quantities in question are generally sized in powers of 2 (e.g. 1024 bits == 2^10), but the binary data is shown in rows of 15 columns. The leading zeros are surprising as well.

So… is there a good reason for this unusual display format?


1 Answer 1


As for the leading zero, I believe the tools are just displaying what's in the ASN.1 as is; the BER/DER encoding rules will insist on a leading 00 byte in some cases.

Specifically, if you encode a positive integer, the msbit of the value stored must be 0 (if it is a 1, the encoded value is assumed to be negative); if the msbyte of the value you want to store has a 1 msbit, then you must prefix a 00 byte byte to the stored value; this 00 byte does not change the value (obviously), but it does make the msbit 0. In both case you have, the most significant nonzero byte has an msbit of 1; hence, they have a 00 prefix.

Now, you state that other components within the private key do not have a 00 prefix; I expect that, if you examine them, the first byte of those values will be somewhere in the 01-7f range, and so don't need a 00 prefix.

  • $\begingroup$ Thanks very much! I'm a bit chagrined to admit that I didn't realize that the DER and PEM formats were based on ASN.1. $\endgroup$
    – Dan Lenski
    Feb 7, 2017 at 6:31
  • 3
    $\begingroup$ Any N bit key with N divisible by 8 will have the MSB set, so the 00 prefix is also very likely. $\endgroup$ Feb 7, 2017 at 9:54
  • 2
    $\begingroup$ The funny thing is that on the other hand in XML-DSIGs CryptoBinary format leading zeros are forbidden (RFC 3275 4.0.1). Nevertheless I have seen some frameworks adding a zero to work around implementations that incorrectly handle this type as twos complement (think of javas BigInteger(KEY_BYTE_ARRAY)) $\endgroup$
    – Drunix
    Feb 7, 2017 at 12:22
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    $\begingroup$ @SimonRichter: to complete that thought, n is usually chosen to be exactly 1024/1536/2048/3072/4096 bits and p and q half that size and thus need the 'extra' sign octet, but d is effectively random in (1,n) and dp dq qinv effectively random in (1,p) or (1,q) so they have about 2/3 or 8/15 probability of not needing it. $\endgroup$ Feb 8, 2017 at 5:39

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