What does OTP prove and what does it not prove about electronic signatures?

In one signatureOTP is a one-time-use code sent to the signer to reduce fraud and increase traceability at the time of agreement; therefore, the use of OTP acts as an authentication layer before or during the act of signing.

The combination of signature and OTP streamlines signature collection because it eliminates in-person steps and, at the same time, creates a verifiable trail (time, channel, attempts, success/failure). Here it's worth separating concepts: authentication is what you do to release the signature (e.g., validate a code); verification is what you do afterward, in a dispute, to check evidence of the process.

Summary

• When is OTP worthwhile (and when does it only add friction) in subscription flows?
• How to configure channel, expiration, retries, and resending without creating loopholes.
• What evidence to keep: logs, timestamps, and audit trail.
• What OTP proves and what it doesn't prove, on its own, in disputes.
• KPIs to monitor completion, failures, signing time, and disputes.

Quick facts

• According to Federal governmentElectronic signatures are recognized in Brazil, and there are specific rules for their use in interactions with public entities.
• According to the ITIAdvanced electronic signatures via gov.br depend on the account security level for access to the Electronic Signature Portal.
• A technical document from ETSI It defines general policy requirements for trusted service providers, governance basis, and controls applicable to subscription-related services.

What is OTP in the context of electronic signatures?

OTP (One-Time Password) is a temporary password, valid for a single use and generally for a short period of time. In electronic signatures, it is often used as proof of ownership of the channel: whoever has access to the email, SMS, or authenticator app can enter the code and proceed with the process.

Therefore, OTP tends to be useful when the goal is to reduce the risk of someone signing in place of someone else without access to the registered channel, as well as creating simple and standardized evidence in the log.

When do subscription and OTP (One-Time Payment) make sense?

OTP makes more sense when the risk is moderate and you need to balance security and conversion. Common examples include: commercial proposals, simple addendums, acceptance terms, and contracts with a relevant average ticket price, but without a certification requirement. If the workflow involves recurring parties, OTP also standardizes validation and reduces rework, which aligns well with efficiency and ROI goals.

In documents with high legal or regulatory impact, an OTP (One-Time Payment) alone is rarely sufficient, and the approach often involves combining factors or using stronger modalities, such as a certificate, according to your risk model. In cases of recurring contractual formalization, a useful basis is understanding... digital contract and what evidence typically supports the case.

Scenario	Typical risk	Does OTP help?	Recommended layers
Terms of acceptance and internal policies	Low	Yes, for traceability.	OTP + audit trail
Standard commercial proposals and contracts	Medium	Yes, with a good setup.	OTP + channel validation + full logs
Sensitive contracts (high value/high exposure)	High	Partial, not alone.	MFA, identity document, timestamp, certificate (when applicable).
Signature with legal requirement/ICP	High	As an extra factor	Certificate + evidence + optional OTP

OTP configuration in steps

A good configuration avoids two problems at once: attrition (which undermines completion) and fragility (which doesn't add up to sufficient evidence). You typically define: delivery channel, expiration time, attempt limit, resending policy, and blocking criteria. In parallel, you decide how the signer regains access when they lose the channel or when their number changes, because this affects the assurance level of the process.

In documents that require organization and governance, it's worthwhile to align with routines of contract management, to standardize rules and reduce exceptions that create loopholes.

1) Channel selection: SMS, email, WhatsApp, or authenticator app.

The channel defines the type of risk. SMS is convenient, but can suffer from SIM card changes, routing issues, and delivery failures.

Email is easy, but it depends on credential hygiene and can be shared in corporate environments. WhatsApp tends to be practical, but it also inherits risks of account hijacking and shared devices, in addition to delivery peculiarities. An authenticator app (TOTP) reduces dependence on carriers and email because it generates codes locally, but it requires registration and user familiarity.

To understand the technical logic of TOTP, the RFC describes that TOTP is an extension of HOTP. by incorporating a time factor in the generation of one-time passwords.

Canal	Operational advantage	Typical risk	When to use
SMS	Low effort for the user.	SIM card replacement and delivery failures	B2C flows and mass subscription
E-mail	Available in almost every scenario.	Shared mailbox, phishing	B2B with personalized emails
WhatsApp	High reading rate	Account hijacking, shared device	Processes with heavy use of mobile
Authenticator App (TOTP)	Network independent at the moment.	Onboarding and support	Environments requiring MFA

2) Define expiration and attempts without creating a loophole.

Short expiration times reduce the window for abuse but increase resubmission and support. Long expiration times reduce friction but extend the time an intercepted code remains useful. A common practice is to calibrate expiration to the actual time of signing on the device (e.g., user opens link, reads, and signs) and limit attempts to reduce brute force attacks. It is also recommended to log failed attempts and apply temporary blocking after a limit to highlight anomalous behavior.

In workflows with PDF signatures, the decision regarding evidence often goes hand in hand with practices such as... Add signature to PDFwhich makes it clear what is recorded in the document and in the records.

3) Auditable control of resending and retrieval.

Resending resolves delivery failures, but can become a vector for abuse if not limited. A simple rule is to allow few resends within a short window, with a cooldown, and log each send with date, time, channel, and result. Recovery (email change, number change, contact update) needs to be treated as a risk event, with additional verification and explicit logging, because it changes control of the channel.

If your operation is already facing "I didn't sign" disputes, one piece of content that helps organize the evidence is... collection of unsigned contract, precisely because it separates allegation, evidence, and procedure.

What evidence should be recorded to make an OTP (Opportunistic Testimony) admissible as evidence?

An OTP (One-Time Payment) only becomes evidence when the process leaves verifiable, consistent, and complete traces. At a minimum, this usually includes: identification of the signer in the flow (name, email, phone number, internal identifier), date and time the OTP was sent, channel used, attempts (success and failure), IP address, user agent, link opening event, authentication event, and signature event.

It is also important to keep track of the link between the validated OTP and the specific document (file hash, version, envelope ID). For document integrity purposes, a practical approach is to understand... hash function in signatureBecause it helps to demonstrate whether there were subsequent changes to the signed file.

Evidence	What proves	Why help in a dispute?
OTP transmission log (date/time/channel)	The challenge was issued.	Shows the beginning of the event and provides context.
Validation log (success/failure/attempts)	That there was possession of the canal and action.	Reduces ambiguity regarding "I did not receive"
IP and user agent	Source of access	It helps identify patterns and fraud.
Time stamp	Verifiable time of the event	Strengthens the chronology of acceptance.
Document Hash/ID	Integrity and versioning	Link the signature to the correct file.

Timestamp and audit trail

When you talk to legal counsel, the discussion almost always comes back to "how do I prove this later?". Timestamps and audit trails answer this question with chronology and integrity. A timestamp helps to pinpoint when something occurred; an audit trail chains events: sending, reading, authentication, signing, downloading, and any rejections.

The specific form of these records varies by platform, but the principle is the same: to allow third parties to reconstruct the flow. An operational point that avoids surprises is to align from the outset on how it works. signature verifier and what data does it display for later validation?

How to combine OTP with other risk-related factors

OTP is a channel ownership factor; it was not designed, on its own, to identify a person with a high degree of security. Therefore, in higher-risk scenarios, the practice is to combine layers: identity data, certificates, biometrics, and MFA. This can be done in stages (only increasing the requirement when the risk increases) or as a minimum standard in certain types of documents.

Authentication and digital identity guidelines often address assurance levels and factors; a technical reference on digital identity is the... NIST SP 800-63-3which structures concepts of guarantee and controls.

OTP + Identity Verification

If you need to associate a subscription with a specific person (and not just the channel), identity verification comes into play: document verification, selfie, biometrics, or integration with databases. In this combination, the OTP validates ownership of the channel at the time of acceptance, while identity verification reduces the risk of fraudulent registration.

In workflows where biometrics are used as a reinforcement, it makes sense to map what qualifies as such. facial biometrics and how evidence is recorded for auditing, avoiding treating selfies as absolute proof without context.

OTP + digital certificate (when applicable)

When a document requires a more robust cryptographic mechanism, the certificate layer changes the level of evidence because it provides specific keys, chain of trust, and validations.

In Brazil, this is often related to ICP-Brasil in various scenarios. In this case, OTP may still be useful as an extra factor (for example, to grant access to the flow), but the probative weight tends to come from the certificate and associated records. To organize the topic, an objective reading is to understand ICP-Brazil and the implications for signature and validation.

What does OTP prove and what does it not prove?

One-Time Pass (OTP) usually proves one thing well: someone with access to the channel (phone, email, or authenticator app) managed to answer the challenge within the defined rules. This is useful, but it has clear limits. It does not, by itself, prove the civil identity of the person who entered the code, nor does it prove intent, capacity, or absence of coercion.

This point frequently comes up in discussions about signature disputes: a signature is a mechanism of acceptance, but the intention can be questioned by context external to the click. A straightforward text on this limitation is that of... Why signatures don't prove intent.which helps to separate technical events from broader legal elements.

Affirmative	Is it true with OTP?	Observation
"The canal was under someone's control."	Yes	It depends on the hygiene of the canal and its recovery policy.
"The person identified was the one who signed."	No, alone	You need identification, certificates, or additional evidence.
"There was free and conscious agreement."	No	Intention and ability depend on context and further evidence.
"The document was not altered after signing."	No, alone	Requires hashing, timestamping, and integrity controls.

How does this relate to legal requirements and standards?

In standards and regulations, the debate tends to be: how to ensure a link with the signatory, control the means of signature, and detect alterations? According to Regulation (EU) 910/2014, the eIDAS It describes that an advanced electronic signature must be uniquely linked to the signer, allow identification, be under exclusive control, and detect changes to the signed data.

In Brazil, the classification for use with public entities appears in Law No. 14.063 / 2020, with rules and procedures. OTP can be part of a security design, but it does not automatically replace requirements that call for stronger identification and control.

Check out these related articles as well:

• Digital signature: how to do it It helps organize the typical steps and evidence of the workflow, from submission to final acceptance.
• Advanced electronic signature It details requirements and scenarios in which extra layers of authentication tend to come into play.
• Legal validity of electronic signature It presents the fundamentals and key points to consider when structuring internal processes.

KPIs to track security and conversion without guesswork.

Without measurement, OTP becomes nothing more than cost and friction. The most useful KPIs combine operation and dispute: completion rate (how many sign up to the end), OTP failure rate (errors, expired, blocked), average signing time (from sending to acceptance), volume of resubmissions, abandonment rate after the OTP stage, and number of disputes per period.

When you cross-reference this data with document type and channel, it becomes easier to determine if the expiration date is too short, if the number of attempts is too low, or if the chosen channel is failing. To connect efficiency to results, the theme of Digital Signature ROI It helps translate time and rework into process impact.

KPI	how to calculate	What does it signal?	Typical action
Completion rate	Subscriptions completed / initiated	General friction in the flow	Adjust UX, channel, and messaging.
OTP failures	OTP failed / OTP sent	Poor delivery, expiration date, confusion.	Recalibrate expiration and attempts
Subscription period	Minutes from submission to acceptance	Operational speed	Optimize steps and notifications
Forwarding by subscription	Total number of forwards / subscriptions	Channel or usability issues	Change channel, limit forwarding, improve instructions.
Disputas	Objections / completed	Legal risk and gaps in evidence	Strengthen authentication trail and layers.

Signature with OTP as a continuous improvement practice

OTP is a tool, not a verdict. It works best when it's part of a clear risk policy, with consistent settings and well-documented evidence. Over time, the data from your own process will tell you whether OTP is reducing disputes, speeding up sign-ups, or simply creating abandonment.

Small adjustments, such as device-appropriate expiration dates, resend limits, and complete logs, often generate quick gains without changing the subscription model. To close the loop, the OTP signature It should be reviewed like any other control: what is being protected, what exceptions exist, and what metrics show that the protection has not become unnecessary friction.

In practical terms, knowing the ZapSign's electronic signature solution It helps visualize how layers such as OTP, audit trail, and metrics can be organized into a single flow.

Frequently Asked Questions (FAQ)

Is OTP in electronic signatures the same as two-factor authentication?

Not necessarily. OTP can be a single factor (channel ownership) used to enable the act of signing. Two-factor authentication (2FA/MFA) combines at least two types of factors, such as ownership (OTP) and something the person knows (password) or something the person is (biometrics). In electronic signatures, OTP often enters as a single step, and becomes MFA only when combined with another additional factor in the same flow.

Does OTP prove that the right person signed?

One-Time Pass (OTP) proves, more directly, that someone with access to the channel (phone, email, or app) was able to respond to the challenge within the defined time. This, in itself, does not equate to proving civil identity, because channels can be shared, compromised, or recovered by third parties. To approximate identity signature, it is common to combine OTP with identity verification, certificates, or stricter recovery policies, in addition to complete records in the audit trail.

What is an appropriate expiration time for OTPs in subscriptions?

It depends on your workflow and channel. A short timeout reduces the window for abuse but increases failures and retransmissions; a long timeout reduces friction but increases risk if the code is intercepted. The safest approach is to calibrate based on the median real-time between link opening and subscription, and review based on metrics such as expiration failures, retransmissions, and abandonment after the OTP step. Changes should be kept in consistent logs for auditing purposes.

Which logs are essential for defending a disputed OTP signature?

Essential logs typically include: OTP transmission (date/time/channel), validation (success/failure/attempts), IP address and user agent, signature event log, document identification (ID and hash/version), and timestamp to tie the timeline. The value of these logs increases when there is a complete audit trail, because it allows third parties to reconstruct the flow. Without these records, OTP tends to become just an operational step, with little evidentiary value.

Can OTP be used in documents with high legal risk?

It can, but as an additional layer, not as the sole control. In high-risk documents, the design usually requires stronger identification, control, and integrity mechanisms, such as digital certificates, robust identity validations and timestamping, as well as formal exception and channel recovery policies. In this scenario, OTP (Online Tracing Protocol) comes in to reduce opportunistic fraud and standardize traceability, provided that its configurations and evidence are aligned with the document's risk level.