cleartext signature: digest determination

[Hauke Laging]

Hello,

in RfC4880 I read this:

https://tools.ietf.org/html/rfc4880#section-7

«If the "Hash" Armor Header is given, the specified message digest    
algorithm(s) are used for the signature.  If there are no such headers, MD5 is 
used.»

That doesn't make sense to me. I checked a cleartext signature with 
gpg --list-packets and got this:

:signature packet: algo 1, keyid 4CB66C1B33FB59FC
        version 4, created 1364174035, md5len 0, sigclass 0x01
        digest algo 2, begin of digest a1 0d
        hashed subpkt 2 len 4 (sig created 2013-03-25)
        subpkt 16 len 8 (issuer key ID 4CB66C1B33FB59FC)
        data: [4093 bits]

This looks like a normal signature packet to me, and it does contain the used 
digest algo. So why should it be necessary to write the used digest into the 
cleartext part? Is that a compatibility issue with older OpenPGP versions? 
Usually that is mentioned but not in the text I quoted.


Hauke
-- 
☺
PGP: 7D82 FB9F D25A 2CE4 5241 6C37 BF4B 8EEF 1A57 1DF5 (seit 2012-11-04)
http://www.openpgp-courses.org/
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