Confirming a Systemic Defect: The ECtHR’s December 2025 Judgments on ByLock and Terrorism Convictions in Türkiye

Mr Coskun Yorulmaz

Coskun is a Turkish lawyer. He has contributed to numerous international reports and articles on the human rights situation in Türkiye. He is a frequent speaker at international conferences on this topic.

In December 2025, the European Court of Human Rights (ECtHR or the Court) delivered a series of judgments concerning terrorism-related offences in Türkiye. These cases were Bozyokuş and Others v. Türkiye (application no. 39586/20 and 131 other applications), Karslı and Others v. Türkiye (no. 18693/20 and 1,435 other applications) and Seyhan and Others v. Türkiye (no. 57837/19 and 851 other applications). In aggregate, 2,420 applicants complained of Türkiye’s violations of Articles 7 (no punishment without law) and/or 6(1) (right to a fair trial) of the European Convention on Human Rights (ECHR or the Convention), in respect of which the ECtHR unanimously found a violation. 

The cases under discussion concerned the applicants’ convictions of the “Gülen Movement,” which the Turkish Government designated as a terrorist organisation after the failed coup attempt of July 2016. Such a designation, however, is a classification adopted within the Turkish legal order and has not been accepted at the international level, on the basis that the attribution of the failed coup attempt and the ensuing terrorism-related offences to the Movement and its members has not been supported by a sufficiently substantiated factual foundation. 

Before that point, the Movement and its members had increasingly criticised certain authorities within the Government, including corruption and abuse of power. They had defended the need for an independent judiciary. Critics and observers have argued that attributing liability to individuals peripherally connected to the Movement suggests a form of political retribution, particularly in the absence of sufficiently substantiated evidence linking the Movement or its members to the events of July 2016. 

In this context, the relevance of the December 2025 judgments lies in the ECtHR’s assessment, which identified a consistent approach to criminal adjudication across a considerable number of cases involving 2,420 applicants. Rather than reassessing the applicants’ individual circumstances, the Court considered whether these applications raised issues similar to those addressed in its previous judgments in Yüksel Yalçınkaya v. Türkiye and Demirhan and Others v. Türkiye. 

Those cases primarily concerned convictions based decisively on the use of the messaging application “ByLock,” which the Turkish courts treated as evidence of membership in the Gülen Movement, and, thereby, as a basis for terrorist affiliation under Article 314(2) of the Turkish Penal Code. ​ByLock was an encrypted communication application that was publicly available before July 2016 and whose downloads, possession, or use were not unlawful under Turkish law at the relevant time. Following the attempted coup, Turkish authorities characterised the application as having been used exclusively by members of the Gülen Movement and treated its use as decisive evidence of organisational membership. The legal significance attributed to this application, therefore, became central to the Court’s assessment. Finding no reasons to depart from its earlier conclusions in these cases, the ECtHR assessed the December judgments on the same legal reasoning, thereby highlighting the unlawful, systematic pattern in which Turkish courts have been attributing criminal liability. 

To clarify the trajectory of the analysis, the discussion begins with the Court’s treatment of ByLock in the Grand Chamber judgment of Yüksel Yalçınkaya v. Türkiye, focusing on its findings under Articles 7 and 6(1) of the Convention. It then traces how that reasoning was reiterated in Demirhan and Others v. Türkiye and subsequently confirmed in the December 2025 judgments. From there, the analysis turns to the systemic implications of this line of case law and its broader consequences for criminal liability grounded in presumed affiliation.

ByLock, Criminal Liability, and the Requirements of Articles 7 and 6(1) of the Convention

ByLock was a digital communication application functionally comparable to widely known messaging services, enabling users to exchange written and audio messages via electronic devices. It was developed by a Turkish-American programmer and launched in 2014, becoming publicly available on prominent digital platforms such as the App Store and Google Play before being withdrawn later that year. Access to and use of ByLock occurred long before the failed coup attempt of July 2016, and at a time when Turkish law did not criminalise its download, possession, or use, nor attach any penal consequences thereto.

According to the Turkish Government in Yüksel Yalçınkaya v. Türkiye, ByLock was not a conventional communication application but was specifically used by members of the Gülen Movement. The national authorities have relied mainly on intelligence material produced by the National Intelligence Agency (MİT) to conclude that the application was designed to facilitate secure internal communication within the Movement to evade law-enforcement surveillance (Yüksel Yalçınkaya §§ 115–117, 124, 158–160, 232). The primary argument advanced by Türkiye has been that ByLock was designed for organisational use and that its technical features indicated its exclusive character, supporting the view that it had an exclusive character (Yüksel Yalçınkaya §§ 115–117, 159–160). On that basis, the mere finding that an individual used ByLock has been regarded as sufficient and decisive evidence of knowing and intentional membership in an armed terrorist organisation under Article 314(2) of the Turkish Penal Code, irrespective of the content of the communications, the purpose of the use, its duration or intensity, or the identity of the interlocutors. (Yüksel Yalçınkaya §§ 232, 258–259). 

Yüksel Yalçınkaya, a former teacher dismissed from the civil service following the state of emergency (Yüksel Yalçınkaya §24), firmly rejected this line of reasoning, distancing himself from any deliberate use of ByLock and emphasising the application’s open availability to the public for a substantial period (Yüksel Yalçınkaya §§ 89–90, 225–226). He further contended that the technical attributes relied upon by the authorities were commonplace rather than exceptional, appearing in many widely used encrypted messaging applications, thereby calling into question the assumption that the use of ByLock automatically denoted organisational affiliation (Yüksel Yalçınkaya §§ 90–339). He also contested the evidentiary soundness of the ByLock data itself, maintaining that the MİT had acquired the data through technical processes that were never disclosed and that he had not been granted access to the material said to demonstrate his use of the application. As a result, he was unable to verify its accuracy, assess the risk of manipulation, or effectively challenge the evidence, which was treated as decisive in his conviction (Yüksel Yalçınkaya §§ 279–282, 286–287, 311, 324–326). 

The ECtHR held that the domestic courts’ approach, under which the alleged use of the ByLock application, standing alone, constituted conclusive proof of membership of an armed terrorist organisation, was incompatible with Articles 7 and 6(1) ECHR. Concerning Article 7 ECHR, the Court found that this approach dispensed with the need to establish the constituent elements of the offence, namely the applicant’s knowledge and intent, thus amounting to an unforeseeable judicial interpretation that retrospectively expanded the scope of criminal liability to the applicant’s detriment (Yüksel Yalçınkaya §§267–272). The Court further emphasised that, at the time of the alleged conduct, neither the use of ByLock nor the applicant’s activities could have enabled him to foresee that such conduct would result in criminal liability for membership of a terrorist organisation (Yüksel Yalçınkaya §§268–270). With regard to Article 6(1) ECHR, the Court held that the proceedings were rendered unfair because the conviction rested decisively on intelligence material obtained by the MİT and Yüksel Yalçınkaya was denied effective access to the underlying data and a genuine opportunity to challenge its reliability, diverging from the principles of equality of arms and adversarial proceedings (Yüksel Yalçınkaya §§311,324–336). The Court ultimately found that his conviction breached both the principle of legality and the right to a fair trial, thereby violating Articles 7 and 6(1) of the Convention (Yüksel Yalçınkaya §§ 272 and 356). 

In Demirhan and Others v. Türkiye, the ECtHR reiterated that the Turkish courts’ categorical approach of treating the mere use of ByLock as decisive proof of membership of an armed terrorist organisation reinforced the systemic problem identified in Yüksel Yalçınkaya v. Türkiye. It also held that this approach was contrary to the object and purpose of Article 7 of the Convention (Demirhan and Others §§ 37–38). The Court further applied the reasoning developed in Yalçınkaya, holding that reliance on untested ByLock evidence, in the absence of adequate procedural safeguards, unduly restricted the rights of the defence and resulted in a violation of the applicants’ right to a fair trial under Article 6(1) (Demirhan and Others §§ 43–48).

This approach was subsequently reaffirmed in the judgments of Bozyokuş and Others, Karslı and Others, and Seyhan and Others, in which the Court again declared that it found no reason to depart from Yalçınkaya, relying on the detailed reasoning set out in Demirhan (inter alia, Bozyokuş § 9; Karslı § 9; Seyhan § 9). 

The Systemic Character of ByLock-Based Criminal Liability

The abovementioned cases reveal that the persistent repetition of violations found by the ECtHR is not the result of mere coincidence, but rather a sign of a broader systemic violation stemming from the national judiciary’s approach to ByLock-related terrorism offences. The ECtHR emphasised in Yüksel Yalçınkaya that the situation that led to a breach of Articles 7 and 6(1) ECHR was not the product of an isolated incident or attributable to the particular turn of events. Still, it may be seen as stemming from a systemic problem inherent in the domestic approach to ByLock-related prosecutions. At the time of its judgment, over 8,000 applications were pending before the Court regarding comparable complaints concerning these provisions for ByLock convictions (Yüksel Yalçınkaya §414).  

In its subsequent decisions, including in Demirhan and Others and the December 2025 judgments, the ECtHR did not reassess the legality of each conviction in isolation and proceeded on the premise that the same domestic method of criminal adjudication was being applied consistently across different courts and proceedings. In the Court’s jurisprudence, a systemic defect is not defined by the recurrence of individual judicial errors, but by the persistent application of an approach that structurally impedes genuine legal assessment. In the ByLock line of cases, criminal responsibility was attributed through an evidentiary framework applied in a largely uniform manner. Once an individual is determined to have used the ByLock application, that determination carries decisive weight in the assessment of liability, serving in practice as sufficient proof of membership of an armed terrorist organisation.

Accordingly, no independent judicial assessment has been undertaken about the constituent elements of the offence, which have been treated as arising automatically from the mere classification of ByLock use. The satisfaction of this minimal threshold thus has predetermined the outcome, leaving the national courts to formalise a result already implied. In an individual case, the use of a single evidentiary factor may seem incidental or susceptible to correction. However, when applied repeatedly across a vast number of proceedings, the same reliance reveals a defect of a different order, one rooted in the interpretative methodology rather than in case-specific judicial error. In such circumstances, when the same evidentiary shortcut is consistently used to establish criminal liability, the issue no longer concerns isolated judicial error but instead indicates a deeper structural problem in the manner in which adjudication is conducted. At a systemic level, the defect does not reside in the particular reasoning of any one decision, but in the interpretative framework through which liability is inferred and judicial reasoning is structured. 

Set against this background, the ramifications of this adjudicative practice transcend the bounds of matters of evidentiary assessment or interpretative method. Criminal liability that is inferred from association rather than established through individualised findings of conduct and intent renders the criminal process particularly vulnerable to influence from the broader political context. In the post-2016 period, individuals connected to the Gülen Movement, even where such a connection was marginal, were exposed to criminal sanction through a system of adjudication that restricted the scope for individualised assessment or rebuttal.

Absent concrete proof connecting individuals to the attempted coup or to acts of violence, the consistent reliance on ByLock use to establish guilt has been understood by observers as indicative of a vindictive approach, thereby enabling criminal law to function as a mechanism for attributing criminal liability based on presumed affiliation rather than demonstrable individual conduct. 

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