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Alexey Avramov
@@ -13,119 +13,22 @@
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# Time-Lock Encryption
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> [!NOTE]
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> `Time cost` value MUST be an integer.
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Find the `Time cost` for a given duration on your hardware.
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Here's how to find the `Time cost` value at which keys are derived with Argon2 within a given time.
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**1. Obtain the average time of one iteration.**
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First, you need to determine the average time of a single Argon2 pass (iteration) on your hardware.
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A. Run with `Time cost` = 100 and measure the total time: `t_total`.
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Try encrypting or decrypting a file.
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For example, create an empty file named `0` and encrypt it with `Time cost` = 100.
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Open the custom options, set `Time cost:` 100, and encrypt file `0`.
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B. Compute the average time per iteration:
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```
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$ touch 0
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$ tird
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MENU
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———————————————————————————————————————————
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0. Exit 1. Info & Warnings
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2. Encrypt 3. Decrypt
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4. Embed 5. Extract
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6. Encrypt & Embed 7. Extract & Decrypt
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8. Create w/ Random 9. Overwrite w/ Random
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———————————————————————————————————————————
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A0. Select an option [0-9]: 2
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I: action #2:
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encrypt file contents and comments;
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write cryptoblob to new file
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C0. Use custom settings? (Y/N, default=N): y
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I: use custom settings: True
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W: decryption will require the same [C1] and [C2] values!
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C1. Time cost (default=4): 100
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I: time cost: 100
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C2. Max padding size, % (default=20):
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I: max padding size, %: 20
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C3. Set fake MAC tag? (Y/N, default=N):
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I: set fake MAC tag: False
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D1. File to encrypt: 0
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I: path: '0'; size: 0 B
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D2. Comments (optional, up to 512 B):
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I: comments will be shown as [None]
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D3. Output (encrypted) file: test1
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I: new empty file 'test1' created
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K1. Keyfile path (optional):
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K2. Passphrase (optional):
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W: no keyfile or passphrase specified!
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I: deriving keys (time-consuming)
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I: keys derived in 87.4s (1m 27.4s)
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I: data size to write: 899 B
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I: reading plaintext, writing cryptoblob
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I: written 100.0%; 899 B in 0.0s; avg 0.2 MiB/s
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I: writing completed; total of 899 B written
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I: pockets (padding) location in output file:
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[16:159] — 143 B
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[703:883] — 180 B
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I: action completed
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t_avg = t_total / 100
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```
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Here we specified the number of Argon2 iterations (`Time cost`):
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**2. Find the `Time cost` for the target duration (in seconds, `seconds_target`).**
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Required number of iterations (`Time cost`):
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```
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C1. Time cost (default=4): 100
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I: time cost: 100
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t_cost = seconds_target / t_avg
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```
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Here we obtained the Argon2 runtime in seconds:
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```
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I: deriving keys (time-consuming)
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I: keys derived in 87.4s (1m 27.4s)
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```
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Find the average time for one pass by dividing total time by the number of iterations:
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```
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t_avg = 87.4 / 100 = 0.874
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```
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So, on my hardware the average time for one iteration was **0.874** seconds.
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### How to find `Time cost` for a given time interval?
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Multiply `t_avg` by the number of seconds in the interval and round to get an integer.
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For one hour:
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```
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t_cost = t_avg * 3600 = 0.874 * 3600 = 3146.4 ≈ 3146
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```
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Thus, `Time cost` = 3146 will make Argon2 run for about one hour on my hardware.
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**Note:** speed can vary depending on CPU and memory load. This is an approximate value.
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For two days:
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```
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t_cost = t_avg * 3600 * 24 * 2 = 0.874 * 3600 * 24 * 2 = 151027.2 ≈ 151027
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```
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**Note:** Measured times may vary with system load; re-run the short benchmark (e.g., `Time cost`=100) a few times and average the results for better accuracy.
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### Approximate `Time cost` for different durations and example `t_avg` values
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|Time|`Time cost`<br>(`t_avg=0.5`)|`Time cost`<br>(`t_avg=1`)|`Time cost`<br>(`t_avg=1.5`)|`Time cost`<br>(`t_avg=2`)|`Time cost`<br>(`t_avg=3`)|`Time cost`<br>(`t_avg=4`)|
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|-|-|-|-|-|-|-|
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|10s|5|10|15|20|30|40|
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|1m (60s)|30|60|90|120|180|240|
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|10m (600s)|300|600|900|1200|1800|2400|
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|1h (3600s)|1800|3600|5400|7200|10800|14400|
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|6h (21600s)|10800|21600|32400|43200|64800|86400|
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|1d (24h, 86400s)|43200|86400|129600|172800|259200|345600|
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|7d (168h, 604800s)|302400|604800|907200|1209600|1814400|2419200|
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|30d (720h, 2592000s)|1296000|2592000|3888000|5184000|7776000|10368000|
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