modulation.rs

`crates/veilvoice-core/src/modulation.rs`

veilvoice-core · 323 lines · read the source here · or on GitHub

Cryptographically-seeded modulation of the effect parameters.

The pitch and formant ratios are never constant: a ChaCha20 CSPRNG picks a new random target every frames_per_target STFT frames, and a one-pole filter glides continuously toward it. Because the transform is therefore non-stationary and unpredictable, an attacker cannot "undo" it by assuming a single fixed shift: there is no single shift to undo, and the target sequence is unknowable without the seed (which never leaves the process and is zeroized on drop).

The seed does not stay put either. It is rolled forward every couple of seconds by default (see Modulator::reseed), so the stream driving any given stretch of audio is closed off permanently once that stretch is past.

In plain words

The amount by which the voice is altered is never held still. It drifts, constantly and unpredictably.

The drift comes from the same kind of random number generator used for encryption, so it cannot be guessed, worked out from what came before, or reproduced by somebody who has the recording. It slides between values rather than jumping, so nothing about it can be heard.

This is what stops the transform from being reversed by anybody who works out the settings, because there is no single setting to work out.

WHAT THIS FILE CONTAINS

323 lines defining 9 functions (5 public), 3 types and 0 constants. Everything below is read out of the source, so it cannot disagree with the code.

The types it owns.

struct Param line 36 · One smoothly-varying parameter bounded to lo, hi.
struct ModValues line 76 · The values handed to the spectral transform for one frame.
struct Modulator line 84 · Non-stationary parameter generator.

What happens when it runs. These are the ways in: public, and nothing else in this file calls them, so they are what an outside caller reaches first.

Modulator::from_seed line 96 · Build from an explicit 32-byte seed (deterministic; used by tests and by session-key-derived seeding).
reaches new
Modulator::fill_phase_offsets line 115 · The 32 fixed per-bin phase offsets consumer needs are derived from the same stream; expose a helper that fills out with values in [0, 2π).
Modulator::reseed line 143 · Roll onto a fresh seed, drawn from the current stream.
Modulator::draw_frames line 164 · Draw a whole number of frames uniformly from lo..=hi.
Modulator::next_frame line 174 · Advance one STFT frame and return the parameters to apply.

WHAT CALLS WHAT

The functions this file defines, and the calls between them. An edge means the callee's name appears, called, inside the caller's body. This is a syntactic reading, not a type-resolved one.

The same graph as Mermaid source

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flowchart TD
    n_new["Param::new<br/>line 47"]
    n_retarget["Param::retarget<br/>line 61"]
    n_step["Param::step<br/>line 68"]
    n_from_seed(["Modulator::from_seed<br/>line 96"])
    n_fill_phase_offsets(["Modulator::fill_phase_offsets<br/>line 115"])
    n_reseed(["Modulator::reseed<br/>line 143"])
    n_draw_frames(["Modulator::draw_frames<br/>line 164"])
    n_next_frame(["Modulator::next_frame<br/>line 174"])
    n_drop["Modulator::drop<br/>line 188"]
    n_from_seed --> n_new
    click n_new href "https://github.com/tilas01/veilvoice/blob/main/crates/veilvoice-core/src/modulation.rs#L47" "open the source"
    click n_retarget href "https://github.com/tilas01/veilvoice/blob/main/crates/veilvoice-core/src/modulation.rs#L61" "open the source"
    click n_step href "https://github.com/tilas01/veilvoice/blob/main/crates/veilvoice-core/src/modulation.rs#L68" "open the source"
    click n_from_seed href "https://github.com/tilas01/veilvoice/blob/main/crates/veilvoice-core/src/modulation.rs#L96" "open the source"
    click n_fill_phase_offsets href "https://github.com/tilas01/veilvoice/blob/main/crates/veilvoice-core/src/modulation.rs#L115" "open the source"
    click n_reseed href "https://github.com/tilas01/veilvoice/blob/main/crates/veilvoice-core/src/modulation.rs#L143" "open the source"
    click n_draw_frames href "https://github.com/tilas01/veilvoice/blob/main/crates/veilvoice-core/src/modulation.rs#L164" "open the source"
    click n_next_frame href "https://github.com/tilas01/veilvoice/blob/main/crates/veilvoice-core/src/modulation.rs#L174" "open the source"
    click n_drop href "https://github.com/tilas01/veilvoice/blob/main/crates/veilvoice-core/src/modulation.rs#L188" "open the source"
    classDef entry fill:#1f2335,stroke:#7aa2f7,color:#c0caf5
    class n_from_seed,n_fill_phase_offsets,n_reseed,n_draw_frames,n_next_frame entry
    classDef helper fill:#1f2335,stroke:#bb9af7,color:#c0caf5
    class n_new,n_retarget,n_step,n_drop helper

This site loads no third-party script, so it cannot run Mermaid; the diagram above is the same nodes and edges drawn by the generator instead. GitHub renders the source below directly.

ITEMS

Item	Line	Documentation
`Param` _struct	36	One smoothly-varying parameter bounded to lo, hi.
`Param::new` _fn	47	Start in the middle of the range, so the first frames are not a slide from an edge the caller never asked for.
`Param::retarget` _fn	61	Draw the next value to move towards.
`Param::step` _fn	68	Move one step of the way towards the target and report where that is.
`ModValues` _{pub struct}	76	The values handed to the spectral transform for one frame.
`Modulator` _{pub struct}	84	Non-stationary parameter generator.
`Modulator::from_seed` _{pub fn}	96	Build from an explicit 32-byte seed (deterministic; used by tests and by session-key-derived seeding).
`Modulator::fill_phase_offsets` _{pub fn}	115	The 32 fixed per-bin phase offsets consumer needs are derived from the same stream; expose a helper that fills out with values in [0, 2π).
`Modulator::reseed` _{pub fn}	143	Roll onto a fresh seed, drawn from the current stream.
`Modulator::draw_frames` _{pub fn}	164	Draw a whole number of frames uniformly from lo..=hi.
`Modulator::next_frame` _{pub fn}	174	Advance one STFT frame and return the parameters to apply.
`Modulator::drop` _fn	188