Bug#678267: Can't preseed encryption cipher
Subject: partman-auto-crypto: Can't preseed encryption cipher
Could support be added for preseeding encryption cipher.
In this moment it's hardcoded in cryptobase.sh / crypto_set_defaults ().
A workaround would be changing cryptobase.sh with early_command: (Thanks to Colin Watson)
d-i partman/early_command string sed -i '/echo aes > \$part\/cipher/s/aes/serpent/' /lib/partman/lib/crypto-base.sh