Cryptography
Knowledge (Mathematics) and Craft (Cryptography) or Profession (Cryptographer) can be used to encrypt or decipher deliberately encrypted written messages. While Linguistics is used to decipher actual information from messages, cryptography is a very deliberate and specific skill. Mathematics skills are always useful, but a character can only receive a bonus from either her skill in craft or her profession.
Using Cryptography
Skill ranks in either mathematics or encryption (either craft or profession) can be combined up to 1½ the number of HD a character has (round down). The character still has to choose to use either Knowledge (Mathematics), Craft (Cryptography) or Profession (Cryptographer) for the actual skill check, however. This means the ability modifier and special bonuses only applies once and only for the skill selected.
A character cannot attempt to decipher a coded message untrained unless she has clear instructions she can follow, and then make an intelligence check assisted by the encryption bonus provided by the instructions.
Example: Exarch the wizard is trying to decipher a message he has intercepted. He is his Circle's cryptographer (Profession (Cryptography)) and is very good at mathematics at level 8. He first adds together his 8 skill ranks in Knowledge (Mathematics) and 8 ranks in Profession (Cryptography), getting a total of 16. However, he has a total of 8 HD, so the maximum skill rank bonus is 12. He chooses to use Knowledge (Mathematics) for the roll, adding his Class Skill bonus and Skill Focus bonus to the roll. His total skill rank bonus (of 12) is then combined with his Intelligence bonus (of 4), Class Skill bonus (of 3) and Skill Focus bonus (of 3) for Knowledge (Mathematics): 12 + 4 + 3 + 3 = 22. He then adds +22 to his decipher roll.
Special: A bard can not substitute a soll to gain bonuses in cryptography.
Choose Encryption
When making an encrypted message, there are a number of techniques one can choose from; each have advantages and disadvantages. If the logistics of ciphers and methods between the sender and recipient is dealt with, the main difference between methods is the time it takes to decipher the message. For the use of rules in Hökaland there are three categories:
- Impossible: One Time Pad or Running Cipher
- Base DC: 20
- These methods are not really used (or needed) by most peope. Many don't even know they exist or how they work.
- Unless a cipher is available, the encryption cannot be broken.
- Takes a very long time to decode, even with cipher and needs notes or other tools.
- Very hard: Polyalphabetic cipher
- Base DC: 15
- Less wieldly and the recipient needs to already have a cipher for effective decryption.
- Messages can be broken without ciphers, but it's hard.
- Takes a long time to decode without cipher.
- Hard: Substitution cipher or Transposition cipher
- Base DC: 5
- Most common encryption of this time period. Makes sure uninitiated can't read it and delays the time it takes for others to do so.
- Messages can be broken without ciphers, but the decoder needs some specialist skill.
- Can be decoded without any additional tools and fairly quickly.
- Code: Simply a matter of writing with replaced words or innuendos. More of a Linguisitc, Bluff or Sense Motive roll than Mathematics, etc.
- Depends on coder's Bluff check.
The character needs to have a total bonus that is higher or equal to the base DC of a cryptography method to use it.
Create Cipher
A cipher is the 'key' to an encryption. It can be used for any number of messages.
To encrypt a message the cryptographer needs to choose a method and make a cipher. When method is chosen she then decides if she wants to modify the base DC further (See table). A character makes an appropriate skill check against the modified DC to see how long time it takes. The creation takes a number of minutes equal to the modified DC of the encryption method, minus 1 for every point rolled over the DC. If the result would reduce the time to between 0 and -5 minutes, it instead takes 30 seconds. If the result reduced the time to less than -5 minutes, it instead takes 6 seconds (1 full-round action).
If she fails the check by 5 or less, her messages will have bits of gibberish in it (GM's discretion) but a fail by more than 5 will just create incoherent messages, even with the cipher to decode it.
Encrypt Message
When a character has made a cipher, she rolls to encrypt a message using this cipher. She makes an appropriate skill check against the modified DC. If she suceeds, she writes the encrypted text in the standard time it would take but if she fails by less than 5, she has made an error and realises she needs to start over. If she fails by more than 5, she doesn't notice the error and the message is altered (at GM's discretion; most likely making a some part undecipherable).
The time it takes to encrypt the message is decided by multipying the modified DC with the length of the message in minutes. Each quarter of a page counts as 1 in this equation, and minimum one quarter page message is calculated for the time. For every point of the skill check result above the DC, the time to encrypt is reduced by 1 minute. If the result would reduce the time to 0 or less, it instead takes 30 seconds.
While writing a message, the character can also combine a cryptography method with additional factor such as reversing the order of the letters in the message or adding an additional substitution cipher. See table. The modified DC is then added with the final modifiers to create the total cipher DC.
Decrypt a Message
Decrypting a message is easy but can take time even if the recipient knows the method and cipher. In the case of Impossible encryption, the recipient needs the cipher written down or on some device showing it.
To see how long time it takes to decrypt a message, add together the skill rank bonus (as described in Using Cryptography) with your other bonuses and roll against the total cipher DC. The time it takes works in the same way as creating a cipher. If the cipher is of an impossible method, the time it takes is calculated like encrypting a message instead.
When decrypting a message while having/knowing the cipher there is no true 'fail'. If the roll fails, the person has spent the calculated time to decrypt it, plus a number of minutes by which she failed and is not yet done. An additional attempt can be started immediately with a circumstantial bonus to the roll of +10. If the roll fails again, the same applies and on the subsequent roll the bonus is +20. This continues until the character has decrypted the message or stops attempting to decrypt the message.
- This will have to change a little as decryption is an incremental process and SOME stuff will be revealed on the first roll
Method Category
Cracking a Cipher
To decrypt a message without the actual cipher (to break the cipher) can be fairly easy but time-consuming, hard or impossible.
- NOTE! Still need to figure out how to deal with failing, since a very complex cipher can be made that IS decipherable!