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ch05: Fixing list item content in 'mnemonic code' section.

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Jonathan Cross 2016-11-11 03:56:40 +01:00
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@ -169,12 +169,12 @@ Mnemonic words are generated automatically by the wallet, using a standardized p
4. Divide the sequence into sections of 11 bits.
5. Map each 11-bit value to a word from the predefined dictionary of 2048 words.
6. The mnemonic code is the sequence of words.
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.Generating entropy and encoding as mnemonic words
image::images/Mnemonic_Words.png["Generating entropy and encoding as mnemonic words"]
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The table <<table_4-5>>, shows the relationship between the size of entropy data and the length of mnemonic codes in words.
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[[table_4-5]]
.Mnemonic codes: entropy and word length
[options="header"]
@ -186,17 +186,16 @@ The table <<table_4-5>>, shows the relationship between the size of entropy data
| 224 | 7 | 231 | 21
| 256 | 8 | 264 | 24
|=======
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[[mnemonic_to_seed]]
===== From Mnemonic to Seed
The mnemonic words represent entropy with a length of 128 to 256 bits. The entropy is then used to derive a longer (512-bit) seed through the use of the key-stretching function PBKDF2. The seed produced is then used to build a deterministic wallet and derive its keys.
**From Mnemonic to Seed**
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The mnemonic words represent entropy with a length of 128 to 256 bits. The entropy is then used to derive a longer (512-bit) seed through the use of the key-stretching function PBKDF2. The seed produced is then used to build a deterministic wallet and derive its keys.
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The key-stretching function takes two parameters: the mnemonic and a _salt_. The purpose of a salt in a key-stretching function is to make it difficult to build a lookup table enabling a brute force attack. In the BIP-39 standard, the salt has another purpose - it allows the introduction of a passphrase which serves as an additional security factor protecting the seed, as we will describe in more detail in <<mnemonic_passphrase>>.
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The process described in steps 7 through 9 below continues from the process described previously in <<generating_mnemonic_words>>.
[start=7]
7. The first parameter to the PBKDF2 key-stretching function is the _mnemonic_ produced from step 6 in <<generating_mnemonic_words>>.
8. The second parameter to the PBKDF2 key-stretching function is a _salt_. The salt is composed of the string constant "+mnemonic+" concatenated with an optional user-supplied passphrase string.
9. PBKDF2 stretches the mnemonic and salt parameters using 2048 rounds of hashing with the HMAC-SHA512 algorithm, producing a 512-bit value as its final output. That 512-bit value is the seed.